MANUSCRIPT

PLANT RESPIRATION KREBS CYCLE FERMENTATION AND

ELECTRON TRANSPORT SYSTEM

Intended to Complete Plant Physiology Subject

ANNISA ADELINA FAJRI

1201304 2012

BIOLOGY EDUCATION (ISTE)

BIOLOGY DEPARTMENT

MATH AND SCIENCE FACULTY

PADANG STATE UNIVERSITY

2013

2

PREFACE

ldquoPlant Respiration Krebs Cycle Fermentation and

Transport Systemrdquo in this manuscript is intended to

complete the task from our lecturer Dr Azwir Anhar

MSi and to meet the needs of university students as

the guidersquos book in learning and practice activity for

the member of International Standard Teacher Education

(ISTE) program in Public University of Padang

My greatest wish is that the readers find this

manuscript helpful in gaining the knowledge they need

for effective learning and practice and in preparing

for examination designed to test that knowledge Reader

comments and suggestions are welcome and have again

been helpful in improving our manuscript Your

contribution may assist many other students in the

coming years

Writer

September 2013

i

__________________________

CONTENT

PREFACE i

CONTENT ii

CHAPTER I INTRODUCTION

A Background 1

B Purposes 2

C Benefits 2

CHAPTER II PLANT RESPIRATION

A Krebs Cycle 3

B Fermentation 8

C Electron Transport System

11

CHAPTER III CLOSING

ii

A Conclution

20

B Suggestion

20

REFERENCE

iii

CHAPTER I

INTRODUCTION

A Background

Connected with photosynthesis a metabolic

process takes place in chloroplast-containing plant

cells which like respiration takes up O2 and

releases CO2 in the light but which contrary to

respiration ceases in the dark This O2CO2 gas

exchange has been called right respiration or

photorespiration The substrate for the

photorespiratory metabolism is again ribulose

biphosphate which can be an acceptor not only for

CO2 but also for O2 By taking up oxygen RuBP is

split into PGA and phospoglycolate The supply of O2

and CO2 regulates the relationship between acceptor

oxidation (photorespiration) and acceptor

carboxylation (photosynthesis) via the enzyme

complex RuBP carboxylaseoxygenase High partial

pressure of O2 favors photorespiration A large

supply of CO2 favors photosynthesis The formation

of phospoglycolate is dependent on the supply of

RuBP via the Calvin cycle the photorespiratory O2

uptake and CO2 release increase with light

intensity

1

The process of glycolate metabolism has not

yet been understood in all details The glycolate is

transported out of the chloroplasts phospoglycolate

and phospate The glycolate is transported out of

the chloroplast into peroxisomes cell compartments

about the size of mitochondria which contain

glycolate oxidase catalase and transaminases In

the peroxysomes when O2 is taken up glycolate is

oxidized to glyoxylate and the peroxide thus

produced is detoxified by catalase Glycolate can

either be completely reduced via oxalate by further

O2 uptake or transformed to glycine by

transamination Glycine is transported from the

peroxisomes into the mitochondria where two

molecules of glycine are coupled to form one

molecule of serine with the release of CO2 Serine

is taken over by the amino acid metabolism or

2

converted to glycerate after deamination by

hydropyruvate This can be photophosporylated in the

chloroplasts and returned to the Calvin cycle or

used elsewhere (Larcher 1980 81-82)

B Purposes

This manuscript is intended to complete the

task from our lecturer Dr Azwir Anhar MSi and

to meet the needs of university students as the

guidersquos book in learning and practice activity for

the member of International Standard Teacher

Education (ISTE) program in Public University of

Padang

C Benefits

Benefits that we gained from this manuscript

are

1 As reference in learning process of plant

phisiology subject especially in plant

respiration title

2 As information about steps in plant

respiration

3

CHAPTER II

PLANT RESPIRATION

A Krebs Cycle

The Krebs cycle was named in honor of the

English biochemist Hans A Krebs who in 1937

proposed a cycle of reactions to explain how pyruvate

breakdown takes place in the breast muscle of

pigeons He called his proposed pathway the citric

acid cycle because citric acid is an important

intermediet Another common name for the same group

of reactions is the Tricarboxuylic Acid (TCA) cycle

a term used because citric and isocitric acids have

three carboxyl groups It was not until the early

1950s that mitochondria capable of carrying out this

cycle were isolated from plant cells

The initial step leading to the Krebs cycle

involves the oxidation and loss of CO2 from pyruvate

and the combination of the remaining 2-carbon acetate

unit with a sulfur-containing compound co-enzyme A

(CoA) forming acetyl CoA This and another

comparable role of CoA in the Krebs cycle are

important reasons why sulfur is an essential element

4

The reaction of pyruvate decarboxylation also

involves a phosporylated form of thiamine (vitamin

B1) as a prostethic group Participation of thiamine

in this section partially explains the essential

function of vitamin B1 in plants and animals Besides

the loss of CO2 two hydrogen atoms are removed from

pyruvic acid during the formation of Acetyl CoA The

enzyme catalyzing the complete reaction is called

pyruvic acid dehydrogenase but it is actually an

organized complex containing numerous copies of three

or four different enzymes The hydrogen atoms removed

are finally accepted by NAD+ yielding NADH

The Krebs cycle accomplishes removal of some of

the electrons from organic acid intermediates and

transfer of these electrons to NAD+ or FAD Notice

that none of dehydrogenase enzyme of the cycle uses

NADP+ as an electron acceptor In fact NADP+ is

usually undetectable in plant mitochondria a

situation opposite to that of chloroplasts where

NADP+ is abundant but where there is much less NAD+

Not only are NADH and FADH2 important products of the

Krebs cycle but one molecule of ATP is formed from

ADP and Pi during the conversion of succinyl coenzyme

A to succinic acid Two additional CO2 molecules are

released in these Krebs cycle reactions so there is

a net loss of both carbon atoms from the incoming

5

acetate of acetyl CoA The release of CO2 in the

Krebs cycle accounts for the product of CO2 in the

summary equation for resspiration but no O2 is

absorbed during any Kreb cycle reaction (Lakitan

2011)

During metabolism the synthesis and breakdown

of different organic compounds takes place through

various pathways like the breakdown and synthesis of

proteins carbohydrates fats and nucleic acids

These different pathways and intermediates are also

responsible for the production of energyKrebrsquos

cycle named after Hans Krebs who began working out

its details in 1930s is a series of reactions in

which the pyruvate from glycolysis is oxidized to Co2

under aerobic conditions Kreb cycle is also known as

citric acid cycle or Tri-carboxylic acid cycle (TCA)

( Galston 1968 76)

6

Steps involved in the process of Kreb cycle are

1 Fate of pyruvic acid

Pyruvic acid can form different compounds by

different pathways ie it can be converted into

lactic acid It can be converted into acetyl

coenzyme A as a result of oxidation

2 Fate of Acetyl CoA

It can either undergo condensation with itself or

its derivatives to form fatty acids having 14 ndash 20

carbons Acetyl CoA can also go through a series

of reactions in Krebs cycle

3 Formation of citrate

Acetyl CoA enzyme condenses with oxaloacetate by

an enzyme citrate synthatase to form citrate with

the release of acetyl CoA If the amount of

7

PADANG STATE UNIVERSITY

2013

2

PREFACE

ldquoPlant Respiration Krebs Cycle Fermentation and

Transport Systemrdquo in this manuscript is intended to

complete the task from our lecturer Dr Azwir Anhar

MSi and to meet the needs of university students as

the guidersquos book in learning and practice activity for

the member of International Standard Teacher Education

(ISTE) program in Public University of Padang

My greatest wish is that the readers find this

manuscript helpful in gaining the knowledge they need

for effective learning and practice and in preparing

for examination designed to test that knowledge Reader

comments and suggestions are welcome and have again

been helpful in improving our manuscript Your

contribution may assist many other students in the

coming years

Writer

September 2013

i

__________________________

CONTENT

PREFACE i

CONTENT ii

CHAPTER I INTRODUCTION

A Background 1

B Purposes 2

C Benefits 2

CHAPTER II PLANT RESPIRATION

A Krebs Cycle 3

B Fermentation 8

C Electron Transport System

11

CHAPTER III CLOSING

ii

A Conclution

20

B Suggestion

20

REFERENCE

iii

CHAPTER I

INTRODUCTION

A Background

Connected with photosynthesis a metabolic

process takes place in chloroplast-containing plant

cells which like respiration takes up O2 and

releases CO2 in the light but which contrary to

respiration ceases in the dark This O2CO2 gas

exchange has been called right respiration or

photorespiration The substrate for the

photorespiratory metabolism is again ribulose

biphosphate which can be an acceptor not only for

CO2 but also for O2 By taking up oxygen RuBP is

split into PGA and phospoglycolate The supply of O2

and CO2 regulates the relationship between acceptor

oxidation (photorespiration) and acceptor

carboxylation (photosynthesis) via the enzyme

complex RuBP carboxylaseoxygenase High partial

pressure of O2 favors photorespiration A large

supply of CO2 favors photosynthesis The formation

of phospoglycolate is dependent on the supply of

RuBP via the Calvin cycle the photorespiratory O2

uptake and CO2 release increase with light

intensity

1

The process of glycolate metabolism has not

yet been understood in all details The glycolate is

transported out of the chloroplasts phospoglycolate

and phospate The glycolate is transported out of

the chloroplast into peroxisomes cell compartments

about the size of mitochondria which contain

glycolate oxidase catalase and transaminases In

the peroxysomes when O2 is taken up glycolate is

oxidized to glyoxylate and the peroxide thus

produced is detoxified by catalase Glycolate can

either be completely reduced via oxalate by further

O2 uptake or transformed to glycine by

transamination Glycine is transported from the

peroxisomes into the mitochondria where two

molecules of glycine are coupled to form one

molecule of serine with the release of CO2 Serine

is taken over by the amino acid metabolism or

2

converted to glycerate after deamination by

hydropyruvate This can be photophosporylated in the

chloroplasts and returned to the Calvin cycle or

used elsewhere (Larcher 1980 81-82)

B Purposes

This manuscript is intended to complete the

task from our lecturer Dr Azwir Anhar MSi and

to meet the needs of university students as the

guidersquos book in learning and practice activity for

the member of International Standard Teacher

Education (ISTE) program in Public University of

Padang

C Benefits

Benefits that we gained from this manuscript

are

1 As reference in learning process of plant

phisiology subject especially in plant

respiration title

2 As information about steps in plant

respiration

3

CHAPTER II

PLANT RESPIRATION

A Krebs Cycle

The Krebs cycle was named in honor of the

English biochemist Hans A Krebs who in 1937

proposed a cycle of reactions to explain how pyruvate

breakdown takes place in the breast muscle of

pigeons He called his proposed pathway the citric

acid cycle because citric acid is an important

intermediet Another common name for the same group

of reactions is the Tricarboxuylic Acid (TCA) cycle

a term used because citric and isocitric acids have

three carboxyl groups It was not until the early

1950s that mitochondria capable of carrying out this

cycle were isolated from plant cells

The initial step leading to the Krebs cycle

involves the oxidation and loss of CO2 from pyruvate

and the combination of the remaining 2-carbon acetate

unit with a sulfur-containing compound co-enzyme A

(CoA) forming acetyl CoA This and another

comparable role of CoA in the Krebs cycle are

important reasons why sulfur is an essential element

4

The reaction of pyruvate decarboxylation also

involves a phosporylated form of thiamine (vitamin

B1) as a prostethic group Participation of thiamine

in this section partially explains the essential

function of vitamin B1 in plants and animals Besides

the loss of CO2 two hydrogen atoms are removed from

pyruvic acid during the formation of Acetyl CoA The

enzyme catalyzing the complete reaction is called

pyruvic acid dehydrogenase but it is actually an

organized complex containing numerous copies of three

or four different enzymes The hydrogen atoms removed

are finally accepted by NAD+ yielding NADH

The Krebs cycle accomplishes removal of some of

the electrons from organic acid intermediates and

transfer of these electrons to NAD+ or FAD Notice

that none of dehydrogenase enzyme of the cycle uses

NADP+ as an electron acceptor In fact NADP+ is

usually undetectable in plant mitochondria a

situation opposite to that of chloroplasts where

NADP+ is abundant but where there is much less NAD+

Not only are NADH and FADH2 important products of the

Krebs cycle but one molecule of ATP is formed from

ADP and Pi during the conversion of succinyl coenzyme

A to succinic acid Two additional CO2 molecules are

released in these Krebs cycle reactions so there is

a net loss of both carbon atoms from the incoming

5

acetate of acetyl CoA The release of CO2 in the

Krebs cycle accounts for the product of CO2 in the

summary equation for resspiration but no O2 is

absorbed during any Kreb cycle reaction (Lakitan

2011)

During metabolism the synthesis and breakdown

of different organic compounds takes place through

various pathways like the breakdown and synthesis of

proteins carbohydrates fats and nucleic acids

These different pathways and intermediates are also

responsible for the production of energyKrebrsquos

cycle named after Hans Krebs who began working out

its details in 1930s is a series of reactions in

which the pyruvate from glycolysis is oxidized to Co2

under aerobic conditions Kreb cycle is also known as

citric acid cycle or Tri-carboxylic acid cycle (TCA)

( Galston 1968 76)

6

Steps involved in the process of Kreb cycle are

1 Fate of pyruvic acid

Pyruvic acid can form different compounds by

different pathways ie it can be converted into

lactic acid It can be converted into acetyl

coenzyme A as a result of oxidation

2 Fate of Acetyl CoA

It can either undergo condensation with itself or

its derivatives to form fatty acids having 14 ndash 20

carbons Acetyl CoA can also go through a series

of reactions in Krebs cycle

3 Formation of citrate

Acetyl CoA enzyme condenses with oxaloacetate by

an enzyme citrate synthatase to form citrate with

the release of acetyl CoA If the amount of

7

PREFACE

ldquoPlant Respiration Krebs Cycle Fermentation and

Transport Systemrdquo in this manuscript is intended to

complete the task from our lecturer Dr Azwir Anhar

MSi and to meet the needs of university students as

the guidersquos book in learning and practice activity for

the member of International Standard Teacher Education

(ISTE) program in Public University of Padang

My greatest wish is that the readers find this

manuscript helpful in gaining the knowledge they need

for effective learning and practice and in preparing

for examination designed to test that knowledge Reader

comments and suggestions are welcome and have again

been helpful in improving our manuscript Your

contribution may assist many other students in the

coming years

Writer

September 2013

i

__________________________

CONTENT

PREFACE i

CONTENT ii

CHAPTER I INTRODUCTION

A Background 1

B Purposes 2

C Benefits 2

CHAPTER II PLANT RESPIRATION

A Krebs Cycle 3

B Fermentation 8

C Electron Transport System

11

CHAPTER III CLOSING

ii

A Conclution

20

B Suggestion

20

REFERENCE

iii

CHAPTER I

INTRODUCTION

A Background

Connected with photosynthesis a metabolic

process takes place in chloroplast-containing plant

cells which like respiration takes up O2 and

releases CO2 in the light but which contrary to

respiration ceases in the dark This O2CO2 gas

exchange has been called right respiration or

photorespiration The substrate for the

photorespiratory metabolism is again ribulose

biphosphate which can be an acceptor not only for

CO2 but also for O2 By taking up oxygen RuBP is

split into PGA and phospoglycolate The supply of O2

and CO2 regulates the relationship between acceptor

oxidation (photorespiration) and acceptor

carboxylation (photosynthesis) via the enzyme

complex RuBP carboxylaseoxygenase High partial

pressure of O2 favors photorespiration A large

supply of CO2 favors photosynthesis The formation

of phospoglycolate is dependent on the supply of

RuBP via the Calvin cycle the photorespiratory O2

uptake and CO2 release increase with light

intensity

1

The process of glycolate metabolism has not

yet been understood in all details The glycolate is

transported out of the chloroplasts phospoglycolate

and phospate The glycolate is transported out of

the chloroplast into peroxisomes cell compartments

about the size of mitochondria which contain

glycolate oxidase catalase and transaminases In

the peroxysomes when O2 is taken up glycolate is

oxidized to glyoxylate and the peroxide thus

produced is detoxified by catalase Glycolate can

either be completely reduced via oxalate by further

O2 uptake or transformed to glycine by

transamination Glycine is transported from the

peroxisomes into the mitochondria where two

molecules of glycine are coupled to form one

molecule of serine with the release of CO2 Serine

is taken over by the amino acid metabolism or

2

converted to glycerate after deamination by

hydropyruvate This can be photophosporylated in the

chloroplasts and returned to the Calvin cycle or

used elsewhere (Larcher 1980 81-82)

B Purposes

This manuscript is intended to complete the

task from our lecturer Dr Azwir Anhar MSi and

to meet the needs of university students as the

guidersquos book in learning and practice activity for

the member of International Standard Teacher

Education (ISTE) program in Public University of

Padang

C Benefits

Benefits that we gained from this manuscript

are

1 As reference in learning process of plant

phisiology subject especially in plant

respiration title

2 As information about steps in plant

respiration

3

CHAPTER II

PLANT RESPIRATION

A Krebs Cycle

The Krebs cycle was named in honor of the

English biochemist Hans A Krebs who in 1937

proposed a cycle of reactions to explain how pyruvate

breakdown takes place in the breast muscle of

pigeons He called his proposed pathway the citric

acid cycle because citric acid is an important

intermediet Another common name for the same group

of reactions is the Tricarboxuylic Acid (TCA) cycle

a term used because citric and isocitric acids have

three carboxyl groups It was not until the early

1950s that mitochondria capable of carrying out this

cycle were isolated from plant cells

The initial step leading to the Krebs cycle

involves the oxidation and loss of CO2 from pyruvate

and the combination of the remaining 2-carbon acetate

unit with a sulfur-containing compound co-enzyme A

(CoA) forming acetyl CoA This and another

comparable role of CoA in the Krebs cycle are

important reasons why sulfur is an essential element

4

The reaction of pyruvate decarboxylation also

involves a phosporylated form of thiamine (vitamin

B1) as a prostethic group Participation of thiamine

in this section partially explains the essential

function of vitamin B1 in plants and animals Besides

the loss of CO2 two hydrogen atoms are removed from

pyruvic acid during the formation of Acetyl CoA The

enzyme catalyzing the complete reaction is called

pyruvic acid dehydrogenase but it is actually an

organized complex containing numerous copies of three

or four different enzymes The hydrogen atoms removed

are finally accepted by NAD+ yielding NADH

The Krebs cycle accomplishes removal of some of

the electrons from organic acid intermediates and

transfer of these electrons to NAD+ or FAD Notice

that none of dehydrogenase enzyme of the cycle uses

NADP+ as an electron acceptor In fact NADP+ is

usually undetectable in plant mitochondria a

situation opposite to that of chloroplasts where

NADP+ is abundant but where there is much less NAD+

Not only are NADH and FADH2 important products of the

Krebs cycle but one molecule of ATP is formed from

ADP and Pi during the conversion of succinyl coenzyme

A to succinic acid Two additional CO2 molecules are

released in these Krebs cycle reactions so there is

a net loss of both carbon atoms from the incoming

5

acetate of acetyl CoA The release of CO2 in the

Krebs cycle accounts for the product of CO2 in the

summary equation for resspiration but no O2 is

absorbed during any Kreb cycle reaction (Lakitan

2011)

During metabolism the synthesis and breakdown

of different organic compounds takes place through

various pathways like the breakdown and synthesis of

proteins carbohydrates fats and nucleic acids

These different pathways and intermediates are also

responsible for the production of energyKrebrsquos

cycle named after Hans Krebs who began working out

its details in 1930s is a series of reactions in

which the pyruvate from glycolysis is oxidized to Co2

under aerobic conditions Kreb cycle is also known as

citric acid cycle or Tri-carboxylic acid cycle (TCA)

( Galston 1968 76)

6

Steps involved in the process of Kreb cycle are

1 Fate of pyruvic acid

Pyruvic acid can form different compounds by

different pathways ie it can be converted into

lactic acid It can be converted into acetyl

coenzyme A as a result of oxidation

2 Fate of Acetyl CoA

It can either undergo condensation with itself or

its derivatives to form fatty acids having 14 ndash 20

carbons Acetyl CoA can also go through a series

of reactions in Krebs cycle

3 Formation of citrate

Acetyl CoA enzyme condenses with oxaloacetate by

an enzyme citrate synthatase to form citrate with

the release of acetyl CoA If the amount of

7

__________________________

CONTENT

PREFACE i

CONTENT ii

CHAPTER I INTRODUCTION

A Background 1

B Purposes 2

C Benefits 2

CHAPTER II PLANT RESPIRATION

A Krebs Cycle 3

B Fermentation 8

C Electron Transport System

11

CHAPTER III CLOSING

ii

A Conclution

20

B Suggestion

20

REFERENCE

iii

CHAPTER I

INTRODUCTION

A Background

Connected with photosynthesis a metabolic

process takes place in chloroplast-containing plant

cells which like respiration takes up O2 and

releases CO2 in the light but which contrary to

respiration ceases in the dark This O2CO2 gas

exchange has been called right respiration or

photorespiration The substrate for the

photorespiratory metabolism is again ribulose

biphosphate which can be an acceptor not only for

CO2 but also for O2 By taking up oxygen RuBP is

split into PGA and phospoglycolate The supply of O2

and CO2 regulates the relationship between acceptor

oxidation (photorespiration) and acceptor

carboxylation (photosynthesis) via the enzyme

complex RuBP carboxylaseoxygenase High partial

pressure of O2 favors photorespiration A large

supply of CO2 favors photosynthesis The formation

of phospoglycolate is dependent on the supply of

RuBP via the Calvin cycle the photorespiratory O2

uptake and CO2 release increase with light

intensity

1

The process of glycolate metabolism has not

yet been understood in all details The glycolate is

transported out of the chloroplasts phospoglycolate

and phospate The glycolate is transported out of

the chloroplast into peroxisomes cell compartments

about the size of mitochondria which contain

glycolate oxidase catalase and transaminases In

the peroxysomes when O2 is taken up glycolate is

oxidized to glyoxylate and the peroxide thus

produced is detoxified by catalase Glycolate can

either be completely reduced via oxalate by further

O2 uptake or transformed to glycine by

transamination Glycine is transported from the

peroxisomes into the mitochondria where two

molecules of glycine are coupled to form one

molecule of serine with the release of CO2 Serine

is taken over by the amino acid metabolism or

2

converted to glycerate after deamination by

hydropyruvate This can be photophosporylated in the

chloroplasts and returned to the Calvin cycle or

used elsewhere (Larcher 1980 81-82)

B Purposes

This manuscript is intended to complete the

task from our lecturer Dr Azwir Anhar MSi and

to meet the needs of university students as the

guidersquos book in learning and practice activity for

the member of International Standard Teacher

Education (ISTE) program in Public University of

Padang

C Benefits

Benefits that we gained from this manuscript

are

1 As reference in learning process of plant

phisiology subject especially in plant

respiration title

2 As information about steps in plant

respiration

3

CHAPTER II

PLANT RESPIRATION

A Krebs Cycle

The Krebs cycle was named in honor of the

English biochemist Hans A Krebs who in 1937

proposed a cycle of reactions to explain how pyruvate

breakdown takes place in the breast muscle of

pigeons He called his proposed pathway the citric

acid cycle because citric acid is an important

intermediet Another common name for the same group

of reactions is the Tricarboxuylic Acid (TCA) cycle

a term used because citric and isocitric acids have

three carboxyl groups It was not until the early

1950s that mitochondria capable of carrying out this

cycle were isolated from plant cells

The initial step leading to the Krebs cycle

involves the oxidation and loss of CO2 from pyruvate

and the combination of the remaining 2-carbon acetate

unit with a sulfur-containing compound co-enzyme A

(CoA) forming acetyl CoA This and another

comparable role of CoA in the Krebs cycle are

important reasons why sulfur is an essential element

4

The reaction of pyruvate decarboxylation also

involves a phosporylated form of thiamine (vitamin

B1) as a prostethic group Participation of thiamine

in this section partially explains the essential

function of vitamin B1 in plants and animals Besides

the loss of CO2 two hydrogen atoms are removed from

pyruvic acid during the formation of Acetyl CoA The

enzyme catalyzing the complete reaction is called

pyruvic acid dehydrogenase but it is actually an

organized complex containing numerous copies of three

or four different enzymes The hydrogen atoms removed

are finally accepted by NAD+ yielding NADH

The Krebs cycle accomplishes removal of some of

the electrons from organic acid intermediates and

transfer of these electrons to NAD+ or FAD Notice

that none of dehydrogenase enzyme of the cycle uses

NADP+ as an electron acceptor In fact NADP+ is

usually undetectable in plant mitochondria a

situation opposite to that of chloroplasts where

NADP+ is abundant but where there is much less NAD+

Not only are NADH and FADH2 important products of the

Krebs cycle but one molecule of ATP is formed from

ADP and Pi during the conversion of succinyl coenzyme

A to succinic acid Two additional CO2 molecules are

released in these Krebs cycle reactions so there is

a net loss of both carbon atoms from the incoming

5

acetate of acetyl CoA The release of CO2 in the

Krebs cycle accounts for the product of CO2 in the

summary equation for resspiration but no O2 is

absorbed during any Kreb cycle reaction (Lakitan

2011)

During metabolism the synthesis and breakdown

of different organic compounds takes place through

various pathways like the breakdown and synthesis of

proteins carbohydrates fats and nucleic acids

These different pathways and intermediates are also

responsible for the production of energyKrebrsquos

cycle named after Hans Krebs who began working out

its details in 1930s is a series of reactions in

which the pyruvate from glycolysis is oxidized to Co2

under aerobic conditions Kreb cycle is also known as

citric acid cycle or Tri-carboxylic acid cycle (TCA)

( Galston 1968 76)

6

Steps involved in the process of Kreb cycle are

1 Fate of pyruvic acid

Pyruvic acid can form different compounds by

different pathways ie it can be converted into

lactic acid It can be converted into acetyl

coenzyme A as a result of oxidation

2 Fate of Acetyl CoA

It can either undergo condensation with itself or

its derivatives to form fatty acids having 14 ndash 20

carbons Acetyl CoA can also go through a series

of reactions in Krebs cycle

3 Formation of citrate

Acetyl CoA enzyme condenses with oxaloacetate by

an enzyme citrate synthatase to form citrate with

the release of acetyl CoA If the amount of

7

A Conclution

20

B Suggestion

20

REFERENCE

iii

CHAPTER I

INTRODUCTION

A Background

Connected with photosynthesis a metabolic

process takes place in chloroplast-containing plant

cells which like respiration takes up O2 and

releases CO2 in the light but which contrary to

respiration ceases in the dark This O2CO2 gas

exchange has been called right respiration or

photorespiration The substrate for the

photorespiratory metabolism is again ribulose

biphosphate which can be an acceptor not only for

CO2 but also for O2 By taking up oxygen RuBP is

split into PGA and phospoglycolate The supply of O2

and CO2 regulates the relationship between acceptor

oxidation (photorespiration) and acceptor

carboxylation (photosynthesis) via the enzyme

complex RuBP carboxylaseoxygenase High partial

pressure of O2 favors photorespiration A large

supply of CO2 favors photosynthesis The formation

of phospoglycolate is dependent on the supply of

RuBP via the Calvin cycle the photorespiratory O2

uptake and CO2 release increase with light

intensity

1

The process of glycolate metabolism has not

yet been understood in all details The glycolate is

transported out of the chloroplasts phospoglycolate

and phospate The glycolate is transported out of

the chloroplast into peroxisomes cell compartments

about the size of mitochondria which contain

glycolate oxidase catalase and transaminases In

the peroxysomes when O2 is taken up glycolate is

oxidized to glyoxylate and the peroxide thus

produced is detoxified by catalase Glycolate can

either be completely reduced via oxalate by further

O2 uptake or transformed to glycine by

transamination Glycine is transported from the

peroxisomes into the mitochondria where two

molecules of glycine are coupled to form one

molecule of serine with the release of CO2 Serine

is taken over by the amino acid metabolism or

2

converted to glycerate after deamination by

hydropyruvate This can be photophosporylated in the

chloroplasts and returned to the Calvin cycle or

used elsewhere (Larcher 1980 81-82)

B Purposes

This manuscript is intended to complete the

task from our lecturer Dr Azwir Anhar MSi and

to meet the needs of university students as the

guidersquos book in learning and practice activity for

the member of International Standard Teacher

Education (ISTE) program in Public University of

Padang

C Benefits

Benefits that we gained from this manuscript

are

1 As reference in learning process of plant

phisiology subject especially in plant

respiration title

2 As information about steps in plant

respiration

3

CHAPTER II

PLANT RESPIRATION

A Krebs Cycle

The Krebs cycle was named in honor of the

English biochemist Hans A Krebs who in 1937

proposed a cycle of reactions to explain how pyruvate

breakdown takes place in the breast muscle of

pigeons He called his proposed pathway the citric

acid cycle because citric acid is an important

intermediet Another common name for the same group

of reactions is the Tricarboxuylic Acid (TCA) cycle

a term used because citric and isocitric acids have

three carboxyl groups It was not until the early

1950s that mitochondria capable of carrying out this

cycle were isolated from plant cells

The initial step leading to the Krebs cycle

involves the oxidation and loss of CO2 from pyruvate

and the combination of the remaining 2-carbon acetate

unit with a sulfur-containing compound co-enzyme A

(CoA) forming acetyl CoA This and another

comparable role of CoA in the Krebs cycle are

important reasons why sulfur is an essential element

4

The reaction of pyruvate decarboxylation also

involves a phosporylated form of thiamine (vitamin

B1) as a prostethic group Participation of thiamine

in this section partially explains the essential

function of vitamin B1 in plants and animals Besides

the loss of CO2 two hydrogen atoms are removed from

pyruvic acid during the formation of Acetyl CoA The

enzyme catalyzing the complete reaction is called

pyruvic acid dehydrogenase but it is actually an

organized complex containing numerous copies of three

or four different enzymes The hydrogen atoms removed

are finally accepted by NAD+ yielding NADH

The Krebs cycle accomplishes removal of some of

the electrons from organic acid intermediates and

transfer of these electrons to NAD+ or FAD Notice

that none of dehydrogenase enzyme of the cycle uses

NADP+ as an electron acceptor In fact NADP+ is

usually undetectable in plant mitochondria a

situation opposite to that of chloroplasts where

NADP+ is abundant but where there is much less NAD+

Not only are NADH and FADH2 important products of the

Krebs cycle but one molecule of ATP is formed from

ADP and Pi during the conversion of succinyl coenzyme

A to succinic acid Two additional CO2 molecules are

released in these Krebs cycle reactions so there is

a net loss of both carbon atoms from the incoming

5

acetate of acetyl CoA The release of CO2 in the

Krebs cycle accounts for the product of CO2 in the

summary equation for resspiration but no O2 is

absorbed during any Kreb cycle reaction (Lakitan

2011)

During metabolism the synthesis and breakdown

of different organic compounds takes place through

various pathways like the breakdown and synthesis of

proteins carbohydrates fats and nucleic acids

These different pathways and intermediates are also

responsible for the production of energyKrebrsquos

cycle named after Hans Krebs who began working out

its details in 1930s is a series of reactions in

which the pyruvate from glycolysis is oxidized to Co2

under aerobic conditions Kreb cycle is also known as

citric acid cycle or Tri-carboxylic acid cycle (TCA)

( Galston 1968 76)

6

Steps involved in the process of Kreb cycle are

1 Fate of pyruvic acid

Pyruvic acid can form different compounds by

different pathways ie it can be converted into

lactic acid It can be converted into acetyl

coenzyme A as a result of oxidation

2 Fate of Acetyl CoA

It can either undergo condensation with itself or

its derivatives to form fatty acids having 14 ndash 20

carbons Acetyl CoA can also go through a series

of reactions in Krebs cycle

3 Formation of citrate

Acetyl CoA enzyme condenses with oxaloacetate by

an enzyme citrate synthatase to form citrate with

the release of acetyl CoA If the amount of

7

CHAPTER I

INTRODUCTION

A Background

Connected with photosynthesis a metabolic

process takes place in chloroplast-containing plant

cells which like respiration takes up O2 and

releases CO2 in the light but which contrary to

respiration ceases in the dark This O2CO2 gas

exchange has been called right respiration or

photorespiration The substrate for the

photorespiratory metabolism is again ribulose

biphosphate which can be an acceptor not only for

CO2 but also for O2 By taking up oxygen RuBP is

split into PGA and phospoglycolate The supply of O2

and CO2 regulates the relationship between acceptor

oxidation (photorespiration) and acceptor

carboxylation (photosynthesis) via the enzyme

complex RuBP carboxylaseoxygenase High partial

pressure of O2 favors photorespiration A large

supply of CO2 favors photosynthesis The formation

of phospoglycolate is dependent on the supply of

RuBP via the Calvin cycle the photorespiratory O2

uptake and CO2 release increase with light

intensity

1

The process of glycolate metabolism has not

yet been understood in all details The glycolate is

transported out of the chloroplasts phospoglycolate

and phospate The glycolate is transported out of

the chloroplast into peroxisomes cell compartments

about the size of mitochondria which contain

glycolate oxidase catalase and transaminases In

the peroxysomes when O2 is taken up glycolate is

oxidized to glyoxylate and the peroxide thus

produced is detoxified by catalase Glycolate can

either be completely reduced via oxalate by further

O2 uptake or transformed to glycine by

transamination Glycine is transported from the

peroxisomes into the mitochondria where two

molecules of glycine are coupled to form one

molecule of serine with the release of CO2 Serine

is taken over by the amino acid metabolism or

2

converted to glycerate after deamination by

hydropyruvate This can be photophosporylated in the

chloroplasts and returned to the Calvin cycle or

used elsewhere (Larcher 1980 81-82)

B Purposes

This manuscript is intended to complete the

task from our lecturer Dr Azwir Anhar MSi and

to meet the needs of university students as the

guidersquos book in learning and practice activity for

the member of International Standard Teacher

Education (ISTE) program in Public University of

Padang

C Benefits

Benefits that we gained from this manuscript

are

1 As reference in learning process of plant

phisiology subject especially in plant

respiration title

2 As information about steps in plant

respiration

3

CHAPTER II

PLANT RESPIRATION

A Krebs Cycle

The Krebs cycle was named in honor of the

English biochemist Hans A Krebs who in 1937

proposed a cycle of reactions to explain how pyruvate

breakdown takes place in the breast muscle of

pigeons He called his proposed pathway the citric

acid cycle because citric acid is an important

intermediet Another common name for the same group

of reactions is the Tricarboxuylic Acid (TCA) cycle

a term used because citric and isocitric acids have

three carboxyl groups It was not until the early

1950s that mitochondria capable of carrying out this

cycle were isolated from plant cells

The initial step leading to the Krebs cycle

involves the oxidation and loss of CO2 from pyruvate

and the combination of the remaining 2-carbon acetate

unit with a sulfur-containing compound co-enzyme A

(CoA) forming acetyl CoA This and another

comparable role of CoA in the Krebs cycle are

important reasons why sulfur is an essential element

4

The reaction of pyruvate decarboxylation also

involves a phosporylated form of thiamine (vitamin

B1) as a prostethic group Participation of thiamine

in this section partially explains the essential

function of vitamin B1 in plants and animals Besides

the loss of CO2 two hydrogen atoms are removed from

pyruvic acid during the formation of Acetyl CoA The

enzyme catalyzing the complete reaction is called

pyruvic acid dehydrogenase but it is actually an

organized complex containing numerous copies of three

or four different enzymes The hydrogen atoms removed

are finally accepted by NAD+ yielding NADH

The Krebs cycle accomplishes removal of some of

the electrons from organic acid intermediates and

transfer of these electrons to NAD+ or FAD Notice

that none of dehydrogenase enzyme of the cycle uses

NADP+ as an electron acceptor In fact NADP+ is

usually undetectable in plant mitochondria a

situation opposite to that of chloroplasts where

NADP+ is abundant but where there is much less NAD+

Not only are NADH and FADH2 important products of the

Krebs cycle but one molecule of ATP is formed from

ADP and Pi during the conversion of succinyl coenzyme

A to succinic acid Two additional CO2 molecules are

released in these Krebs cycle reactions so there is

a net loss of both carbon atoms from the incoming

5

acetate of acetyl CoA The release of CO2 in the

Krebs cycle accounts for the product of CO2 in the

summary equation for resspiration but no O2 is

absorbed during any Kreb cycle reaction (Lakitan

2011)

During metabolism the synthesis and breakdown

of different organic compounds takes place through

various pathways like the breakdown and synthesis of

proteins carbohydrates fats and nucleic acids

These different pathways and intermediates are also

responsible for the production of energyKrebrsquos

cycle named after Hans Krebs who began working out

its details in 1930s is a series of reactions in

which the pyruvate from glycolysis is oxidized to Co2

under aerobic conditions Kreb cycle is also known as

citric acid cycle or Tri-carboxylic acid cycle (TCA)

( Galston 1968 76)

6

Steps involved in the process of Kreb cycle are

1 Fate of pyruvic acid

Pyruvic acid can form different compounds by

different pathways ie it can be converted into

lactic acid It can be converted into acetyl

coenzyme A as a result of oxidation

2 Fate of Acetyl CoA

It can either undergo condensation with itself or

its derivatives to form fatty acids having 14 ndash 20

carbons Acetyl CoA can also go through a series

of reactions in Krebs cycle

3 Formation of citrate

Acetyl CoA enzyme condenses with oxaloacetate by

an enzyme citrate synthatase to form citrate with

the release of acetyl CoA If the amount of

7

The process of glycolate metabolism has not

yet been understood in all details The glycolate is

transported out of the chloroplasts phospoglycolate

and phospate The glycolate is transported out of

the chloroplast into peroxisomes cell compartments

about the size of mitochondria which contain

glycolate oxidase catalase and transaminases In

the peroxysomes when O2 is taken up glycolate is

oxidized to glyoxylate and the peroxide thus

produced is detoxified by catalase Glycolate can

either be completely reduced via oxalate by further

O2 uptake or transformed to glycine by

transamination Glycine is transported from the

peroxisomes into the mitochondria where two

molecules of glycine are coupled to form one

molecule of serine with the release of CO2 Serine

is taken over by the amino acid metabolism or

2

converted to glycerate after deamination by

hydropyruvate This can be photophosporylated in the

chloroplasts and returned to the Calvin cycle or

used elsewhere (Larcher 1980 81-82)

B Purposes

This manuscript is intended to complete the

task from our lecturer Dr Azwir Anhar MSi and

to meet the needs of university students as the

guidersquos book in learning and practice activity for

the member of International Standard Teacher

Education (ISTE) program in Public University of

Padang

C Benefits

Benefits that we gained from this manuscript

are

1 As reference in learning process of plant

phisiology subject especially in plant

respiration title

2 As information about steps in plant

respiration

3

CHAPTER II

PLANT RESPIRATION

A Krebs Cycle

The Krebs cycle was named in honor of the

English biochemist Hans A Krebs who in 1937

proposed a cycle of reactions to explain how pyruvate

breakdown takes place in the breast muscle of

pigeons He called his proposed pathway the citric

acid cycle because citric acid is an important

intermediet Another common name for the same group

of reactions is the Tricarboxuylic Acid (TCA) cycle

a term used because citric and isocitric acids have

three carboxyl groups It was not until the early

1950s that mitochondria capable of carrying out this

cycle were isolated from plant cells

The initial step leading to the Krebs cycle

involves the oxidation and loss of CO2 from pyruvate

and the combination of the remaining 2-carbon acetate

unit with a sulfur-containing compound co-enzyme A

(CoA) forming acetyl CoA This and another

comparable role of CoA in the Krebs cycle are

important reasons why sulfur is an essential element

4

The reaction of pyruvate decarboxylation also

involves a phosporylated form of thiamine (vitamin

B1) as a prostethic group Participation of thiamine

in this section partially explains the essential

function of vitamin B1 in plants and animals Besides

the loss of CO2 two hydrogen atoms are removed from

pyruvic acid during the formation of Acetyl CoA The

enzyme catalyzing the complete reaction is called

pyruvic acid dehydrogenase but it is actually an

organized complex containing numerous copies of three

or four different enzymes The hydrogen atoms removed

are finally accepted by NAD+ yielding NADH

The Krebs cycle accomplishes removal of some of

the electrons from organic acid intermediates and

transfer of these electrons to NAD+ or FAD Notice

that none of dehydrogenase enzyme of the cycle uses

NADP+ as an electron acceptor In fact NADP+ is

usually undetectable in plant mitochondria a

situation opposite to that of chloroplasts where

NADP+ is abundant but where there is much less NAD+

Not only are NADH and FADH2 important products of the

Krebs cycle but one molecule of ATP is formed from

ADP and Pi during the conversion of succinyl coenzyme

A to succinic acid Two additional CO2 molecules are

released in these Krebs cycle reactions so there is

a net loss of both carbon atoms from the incoming

5

acetate of acetyl CoA The release of CO2 in the

Krebs cycle accounts for the product of CO2 in the

summary equation for resspiration but no O2 is

absorbed during any Kreb cycle reaction (Lakitan

2011)

During metabolism the synthesis and breakdown

of different organic compounds takes place through

various pathways like the breakdown and synthesis of

proteins carbohydrates fats and nucleic acids

These different pathways and intermediates are also

responsible for the production of energyKrebrsquos

cycle named after Hans Krebs who began working out

its details in 1930s is a series of reactions in

which the pyruvate from glycolysis is oxidized to Co2

under aerobic conditions Kreb cycle is also known as

citric acid cycle or Tri-carboxylic acid cycle (TCA)

( Galston 1968 76)

6

Steps involved in the process of Kreb cycle are

1 Fate of pyruvic acid

Pyruvic acid can form different compounds by

different pathways ie it can be converted into

lactic acid It can be converted into acetyl

coenzyme A as a result of oxidation

2 Fate of Acetyl CoA

It can either undergo condensation with itself or

its derivatives to form fatty acids having 14 ndash 20

carbons Acetyl CoA can also go through a series

of reactions in Krebs cycle

3 Formation of citrate

Acetyl CoA enzyme condenses with oxaloacetate by

an enzyme citrate synthatase to form citrate with

the release of acetyl CoA If the amount of

7

converted to glycerate after deamination by

hydropyruvate This can be photophosporylated in the

chloroplasts and returned to the Calvin cycle or

used elsewhere (Larcher 1980 81-82)

B Purposes

This manuscript is intended to complete the

task from our lecturer Dr Azwir Anhar MSi and

to meet the needs of university students as the

guidersquos book in learning and practice activity for

the member of International Standard Teacher

Education (ISTE) program in Public University of

Padang

C Benefits

Benefits that we gained from this manuscript

are

1 As reference in learning process of plant

phisiology subject especially in plant

respiration title

2 As information about steps in plant

respiration

3

CHAPTER II

PLANT RESPIRATION

A Krebs Cycle

The Krebs cycle was named in honor of the

English biochemist Hans A Krebs who in 1937

proposed a cycle of reactions to explain how pyruvate

breakdown takes place in the breast muscle of

pigeons He called his proposed pathway the citric

acid cycle because citric acid is an important

intermediet Another common name for the same group

of reactions is the Tricarboxuylic Acid (TCA) cycle

a term used because citric and isocitric acids have

three carboxyl groups It was not until the early

1950s that mitochondria capable of carrying out this

cycle were isolated from plant cells

The initial step leading to the Krebs cycle

involves the oxidation and loss of CO2 from pyruvate

and the combination of the remaining 2-carbon acetate

unit with a sulfur-containing compound co-enzyme A

(CoA) forming acetyl CoA This and another

comparable role of CoA in the Krebs cycle are

important reasons why sulfur is an essential element

4

The reaction of pyruvate decarboxylation also

involves a phosporylated form of thiamine (vitamin

B1) as a prostethic group Participation of thiamine

in this section partially explains the essential

function of vitamin B1 in plants and animals Besides

the loss of CO2 two hydrogen atoms are removed from

pyruvic acid during the formation of Acetyl CoA The

enzyme catalyzing the complete reaction is called

pyruvic acid dehydrogenase but it is actually an

organized complex containing numerous copies of three

or four different enzymes The hydrogen atoms removed

are finally accepted by NAD+ yielding NADH

The Krebs cycle accomplishes removal of some of

the electrons from organic acid intermediates and

transfer of these electrons to NAD+ or FAD Notice

that none of dehydrogenase enzyme of the cycle uses

NADP+ as an electron acceptor In fact NADP+ is

usually undetectable in plant mitochondria a

situation opposite to that of chloroplasts where

NADP+ is abundant but where there is much less NAD+

Not only are NADH and FADH2 important products of the

Krebs cycle but one molecule of ATP is formed from

ADP and Pi during the conversion of succinyl coenzyme

A to succinic acid Two additional CO2 molecules are

released in these Krebs cycle reactions so there is

a net loss of both carbon atoms from the incoming

5

acetate of acetyl CoA The release of CO2 in the

Krebs cycle accounts for the product of CO2 in the

summary equation for resspiration but no O2 is

absorbed during any Kreb cycle reaction (Lakitan

2011)

During metabolism the synthesis and breakdown

of different organic compounds takes place through

various pathways like the breakdown and synthesis of

proteins carbohydrates fats and nucleic acids

These different pathways and intermediates are also

responsible for the production of energyKrebrsquos

cycle named after Hans Krebs who began working out

its details in 1930s is a series of reactions in

which the pyruvate from glycolysis is oxidized to Co2

under aerobic conditions Kreb cycle is also known as

citric acid cycle or Tri-carboxylic acid cycle (TCA)

( Galston 1968 76)

6

Steps involved in the process of Kreb cycle are

1 Fate of pyruvic acid

Pyruvic acid can form different compounds by

different pathways ie it can be converted into

lactic acid It can be converted into acetyl

coenzyme A as a result of oxidation

2 Fate of Acetyl CoA

It can either undergo condensation with itself or

its derivatives to form fatty acids having 14 ndash 20

carbons Acetyl CoA can also go through a series

of reactions in Krebs cycle

3 Formation of citrate

Acetyl CoA enzyme condenses with oxaloacetate by

an enzyme citrate synthatase to form citrate with

the release of acetyl CoA If the amount of

7

CHAPTER II

PLANT RESPIRATION

A Krebs Cycle

The Krebs cycle was named in honor of the

English biochemist Hans A Krebs who in 1937

proposed a cycle of reactions to explain how pyruvate

breakdown takes place in the breast muscle of

pigeons He called his proposed pathway the citric

acid cycle because citric acid is an important

intermediet Another common name for the same group

of reactions is the Tricarboxuylic Acid (TCA) cycle

a term used because citric and isocitric acids have

three carboxyl groups It was not until the early

1950s that mitochondria capable of carrying out this

cycle were isolated from plant cells

The initial step leading to the Krebs cycle

involves the oxidation and loss of CO2 from pyruvate

and the combination of the remaining 2-carbon acetate

unit with a sulfur-containing compound co-enzyme A

(CoA) forming acetyl CoA This and another

comparable role of CoA in the Krebs cycle are

important reasons why sulfur is an essential element

4

The reaction of pyruvate decarboxylation also

involves a phosporylated form of thiamine (vitamin

B1) as a prostethic group Participation of thiamine

in this section partially explains the essential

function of vitamin B1 in plants and animals Besides

the loss of CO2 two hydrogen atoms are removed from

pyruvic acid during the formation of Acetyl CoA The

enzyme catalyzing the complete reaction is called

pyruvic acid dehydrogenase but it is actually an

organized complex containing numerous copies of three

or four different enzymes The hydrogen atoms removed

are finally accepted by NAD+ yielding NADH

The Krebs cycle accomplishes removal of some of

the electrons from organic acid intermediates and

transfer of these electrons to NAD+ or FAD Notice

that none of dehydrogenase enzyme of the cycle uses

NADP+ as an electron acceptor In fact NADP+ is

usually undetectable in plant mitochondria a

situation opposite to that of chloroplasts where

NADP+ is abundant but where there is much less NAD+

Not only are NADH and FADH2 important products of the

Krebs cycle but one molecule of ATP is formed from

ADP and Pi during the conversion of succinyl coenzyme

A to succinic acid Two additional CO2 molecules are

released in these Krebs cycle reactions so there is

a net loss of both carbon atoms from the incoming

5

acetate of acetyl CoA The release of CO2 in the

Krebs cycle accounts for the product of CO2 in the

summary equation for resspiration but no O2 is

absorbed during any Kreb cycle reaction (Lakitan

2011)

During metabolism the synthesis and breakdown

of different organic compounds takes place through

various pathways like the breakdown and synthesis of

proteins carbohydrates fats and nucleic acids

These different pathways and intermediates are also

responsible for the production of energyKrebrsquos

cycle named after Hans Krebs who began working out

its details in 1930s is a series of reactions in

which the pyruvate from glycolysis is oxidized to Co2

under aerobic conditions Kreb cycle is also known as

citric acid cycle or Tri-carboxylic acid cycle (TCA)

( Galston 1968 76)

6

Steps involved in the process of Kreb cycle are

1 Fate of pyruvic acid

Pyruvic acid can form different compounds by

different pathways ie it can be converted into

lactic acid It can be converted into acetyl

coenzyme A as a result of oxidation

2 Fate of Acetyl CoA

It can either undergo condensation with itself or

its derivatives to form fatty acids having 14 ndash 20

carbons Acetyl CoA can also go through a series

of reactions in Krebs cycle

3 Formation of citrate

Acetyl CoA enzyme condenses with oxaloacetate by

an enzyme citrate synthatase to form citrate with

the release of acetyl CoA If the amount of

7

The reaction of pyruvate decarboxylation also

involves a phosporylated form of thiamine (vitamin

B1) as a prostethic group Participation of thiamine

in this section partially explains the essential

function of vitamin B1 in plants and animals Besides

the loss of CO2 two hydrogen atoms are removed from

pyruvic acid during the formation of Acetyl CoA The

enzyme catalyzing the complete reaction is called

pyruvic acid dehydrogenase but it is actually an

organized complex containing numerous copies of three

or four different enzymes The hydrogen atoms removed

are finally accepted by NAD+ yielding NADH

The Krebs cycle accomplishes removal of some of

the electrons from organic acid intermediates and

transfer of these electrons to NAD+ or FAD Notice

that none of dehydrogenase enzyme of the cycle uses

NADP+ as an electron acceptor In fact NADP+ is

usually undetectable in plant mitochondria a

situation opposite to that of chloroplasts where

NADP+ is abundant but where there is much less NAD+

Not only are NADH and FADH2 important products of the

Krebs cycle but one molecule of ATP is formed from

ADP and Pi during the conversion of succinyl coenzyme

A to succinic acid Two additional CO2 molecules are

released in these Krebs cycle reactions so there is

a net loss of both carbon atoms from the incoming

5

acetate of acetyl CoA The release of CO2 in the

Krebs cycle accounts for the product of CO2 in the

summary equation for resspiration but no O2 is

absorbed during any Kreb cycle reaction (Lakitan

2011)

During metabolism the synthesis and breakdown

of different organic compounds takes place through

various pathways like the breakdown and synthesis of

proteins carbohydrates fats and nucleic acids

These different pathways and intermediates are also

responsible for the production of energyKrebrsquos

cycle named after Hans Krebs who began working out

its details in 1930s is a series of reactions in

which the pyruvate from glycolysis is oxidized to Co2

under aerobic conditions Kreb cycle is also known as

citric acid cycle or Tri-carboxylic acid cycle (TCA)

( Galston 1968 76)

6

Steps involved in the process of Kreb cycle are

1 Fate of pyruvic acid

Pyruvic acid can form different compounds by

different pathways ie it can be converted into

lactic acid It can be converted into acetyl

coenzyme A as a result of oxidation

2 Fate of Acetyl CoA

It can either undergo condensation with itself or

its derivatives to form fatty acids having 14 ndash 20

carbons Acetyl CoA can also go through a series

of reactions in Krebs cycle

3 Formation of citrate

Acetyl CoA enzyme condenses with oxaloacetate by

an enzyme citrate synthatase to form citrate with

the release of acetyl CoA If the amount of

7

acetate of acetyl CoA The release of CO2 in the

Krebs cycle accounts for the product of CO2 in the

summary equation for resspiration but no O2 is

absorbed during any Kreb cycle reaction (Lakitan

2011)

During metabolism the synthesis and breakdown

of different organic compounds takes place through

various pathways like the breakdown and synthesis of

proteins carbohydrates fats and nucleic acids

These different pathways and intermediates are also

responsible for the production of energyKrebrsquos

cycle named after Hans Krebs who began working out

its details in 1930s is a series of reactions in

which the pyruvate from glycolysis is oxidized to Co2

under aerobic conditions Kreb cycle is also known as

citric acid cycle or Tri-carboxylic acid cycle (TCA)

( Galston 1968 76)

6

Steps involved in the process of Kreb cycle are

1 Fate of pyruvic acid

Pyruvic acid can form different compounds by

different pathways ie it can be converted into

lactic acid It can be converted into acetyl

coenzyme A as a result of oxidation

2 Fate of Acetyl CoA

It can either undergo condensation with itself or

its derivatives to form fatty acids having 14 ndash 20

carbons Acetyl CoA can also go through a series

of reactions in Krebs cycle

3 Formation of citrate

Acetyl CoA enzyme condenses with oxaloacetate by

an enzyme citrate synthatase to form citrate with

the release of acetyl CoA If the amount of

7

Steps involved in the process of Kreb cycle are

1 Fate of pyruvic acid

Pyruvic acid can form different compounds by

different pathways ie it can be converted into

lactic acid It can be converted into acetyl

coenzyme A as a result of oxidation

2 Fate of Acetyl CoA

It can either undergo condensation with itself or

its derivatives to form fatty acids having 14 ndash 20

carbons Acetyl CoA can also go through a series

of reactions in Krebs cycle

3 Formation of citrate

Acetyl CoA enzyme condenses with oxaloacetate by

an enzyme citrate synthatase to form citrate with

the release of acetyl CoA If the amount of

7

oxaloacetate is very small then small number of

acetyl CoA would be reacting with oxaloacetate

leaving surplus acetyl CoA to go through another

pathway for the formation of long chain fatty

acids

4 Formation of C is ndash acotinate and iso-citrate

Citrate is changed first into C is ndash acotinate and

then to iso-citrate under the enzyme acotinase

Equilibrium is established between citrate C is

aconitate and iso-citrate It has been observed

that most of the time this equilibrium is shifted

towards the iso-citrate If the concentration of

iso-citrate in increased the formation of citrate

will result which also indicates that the

equilibrium also shift in the reserve direction

5 Formation of oxalosucinate

Iso-citrate is acted upon by an enzyme iso-citrate

dehydrogenase using nicotinamide adenine

dinucleotide (NAD) as coenzyme As a result iso-

citrate is converted into oxalosucciate and NAD is

reduced to NADH2 Similar reaction is carried out

by same enzyme using nicotindmide adenine

dinucleotide phosphate (NADP) as coenzyme which is

reduced to NDAPH2

6 Formation of α-Ketoglutarate

8

Oxalosucciante is changed into α-ketoglutrate by

iso-citrate dehydrogenase with the help of

coenzyme NAD or NADP In this reaction carbon

dioxide and NADH2 or NADPH2 are also released

7 Formation of succinyl CoA

α-ketoglutarate combines with acetyl CoA in the

presence of coenzyme NAD and enzyme α-

keoglutaratedehyrogenase to form succinyl coenzyme

a carbon dioxide and NADH2

8 Formation of Succinate

Later on coenzyme a is removed from succinyl

coenzyme A in the presence of guanosinediphosphate

(GDP) and inorganic phosphate to form succinate

and guanosine triphosphate (GTP) This reaction is

carried out by an enzyme called succinyl CoA

synthatase

9 Formation of Funarate

An enzyme succinic dehydrogenase removes hydrogen

from succinate to form funarate

10 Formation of Malate

Fumarate reacts with water in the presence of

enzyme fumarase to form malate

11 Regeneration of oxaloacetate

Malate is oxidized by malic dehydrogenase and NAD

forming oxaloacetate and NADH2 Thus oxalo acetate

is again available to start another cycle

9

(httpkrebs-cycle-or-citric-acid-cycle-or-

trihtml)

Importance of CTA Cycle

1 Source of energy

In addition to routine organic compounds described

above by products like nicotinanide adenine

dinucleotide (NADH2) and guanosine triphosphate

(GTP) are the source of biological energy NADH2

after oxidation produce energy whereas GTP is

itself high energy phosphate compound

2 Oxidation of organic compounds taken as food

Oxidation of fats carbohydrates and proteins take

place through it or in other words it can be said

that oxidation of all compounds having carbon

atoms can take place through TCA cycle Some of

amino acids like alanine glutamic acid and

aspartic acid at one stage or the other enters

into TCA cycle eg glutamic acid enters cycle

after its transformation into α-ketoglutarate

Similarly alanine enters the cycle after its

conversion into pyruvate

3 Intermediate compounds

TCA cycle is also involved in synthesis of

intermediate compounds leading to the formation of

larger molecules (Ray 1963 17)

10

Verification of Krebs Cycle

It was done by radioactive traces like C14 as

radioactive carbon dioxide of different levels and

reactions After addition of radioactive carbon

dioxide different chemical compounds produced like

glucose fats amino acids were isolated and looked

for radioactive carbon In this way whole of

metabolic reaction were verified including individual

reactions alternative metabolic pathways

intermediates of fats carbohydrates and amino acids

etc in the body cells as well as in test tubes

(Wilson 1962 99)

The primary function of the Krebs cycle are as

follows

1 Reduction of NAD+ and FAD to the electron donors

NADH and FADH2 that are subsequently oxidized to

yield ATP

2 Direct synthesis of a limited amount of ATP (1 ATP

for each pyruvate oxidized)

3 Formation of carbon skeletons that can be used to

synthesize certain amino acids that in turn are

converted into larger molecules

11

B Fermentation

Although glycolysis can function well without

O2 the further oxidation of pyruvate and NADH by

mitochondria requires this gas Thus when O2 is

limiting NADH and pyruvate begin to accumulate

Under this condition plants carry out fermentation

(anaerobic respiration) forming either ethanol or

lactic acid (usually ethanol) As shown in the

picture below the two top reactions consists of a

decarboxylation to form acetaldehide then rapid

reduction of acetaldehide by NADH to form ethanol

These reactions are catalyzed by alcohol

dehydrogenase Some cells contain lactic acid

12

dehydrogenase which uses NADH to reduce pyruvic to

lactic acid Thus either ethanol or lactic acid or

both are fermentation products depending on the

activities of each dehydrogenase present In each

case NADH is the reductant and only under

anaerobic conditions is it abundant anough to cause

reduction Furthermore in some plants NADH is used

to cause accumulation of the other compounds when O2

is limiting especially malate and glycerol

(Salisbury 1969 233)

The fate of pyruvate produced by glycolysis

depends primarily on whether or not oxygen is

present Under normal aerobic conditions pyruvate

is transported into the mitochondrion where it is

further oxidized to CO2 and water transferring its

electrons ultimately to molecular oxygen

13

Although higher plants are obligate aerobs and

are able to tolerate anoxia for only short periods

tissues or organs are occasionally subjected to

anaerobic conditions A typical siuation is that of

roots when the soil is saturated with water When

there is no oxygen to serve as the terminal electron

acceptor mitochondrial respiration will shut down

and metabolism will shift over to fermentation

Fermentation converts pyruvate to either ethanol

through the action of the enzyme alcohol

dehydrogenase (ADH) or lactate via lactate

dehydrogenase (LDH) In most plants the principal

products of fermentation are CO2 and ethanol Some

lactate may be formed primarily in the early stages

of anoxia However lactate lowers he pH of the

cytosol which in turn activates pyruvate

decarboxylase and initiates the production of

ethanol

Either one of the fermentation reactions

consumes the NADH produced earlier in glycolysis by

the oxidation of glyceraldehide-3-P Although this

means there is no net gain of reducing potential in

fermentation This recycling of NADH is still

important to the cell The poolof NADH plus NAD+ in

the cell is relatively small and if the NADH is not

recycled there will be no supply of NAD+ to support

14

the continued oxidation of glyceraldehyde-3-P if

this were the case glycolysis and the producton of

even the small quantities of ATP necessary to

maintain the cells under anaerobic conditions would

then grind to a halt (Hopkins 2009 241)

Soil waterlogging has long been identified as a

major abiotic stress and the constraints that it

provides the roots have a significant effect on the

growth and development of plants When this event

occurs in the spring then these puddles can reduce

seed germination and seedling development Thus a

pool of water is an important factor that affects

the growth development and survival of plant

species not only in natural ecosystems but also

the agricultural and horticultural systems (Dat et

al 2006)

After flooding rapid changes in soil

properties At the time of pore water meets the

land the air pushed out gas diffusion is reduced

and toxic compounds accumulate due to anaerobic

conditions All of these changes greatly affect the

ability of plants to survive In response increased

stomatal resistance photosynthesis and root

hydraulic conductivity decreases and reduced

translocation fotoassimilat Nevertheless one of

15

the best adaptations of plants to hypoxia anoxia

is transitional biochemical and metabolic processes

common to current limited availability of O2 (Dat et

al 2004)

Selective synthesis of a set of about 20

anaerobic stress proteins (ANPS) allows the process

of producing energy without oxygen metabolism under

anaerobic conditions (Subbaiah and Sachs 2003)

Another adaptation is the observed morphological

changes consisting of the formation of hypertrophic

lenticels adventitious root initiation and or

progression of aerenchyma (Vartapetian and Jackson

1997 Jackson and Colmer 2005 Folzer et al 2006)

This review details the diverse plant stress

response to hypoxia anoxia caused by soil

waterlogging flooding and examines some of the key

features of metabolic adaptation physiological and

morphological

16

C Electron

Transport System

We noted earlier that one of the principal

functions of the respiration is to retrieve in

useful form some of the energy initially stored in

assimilates Our traditional measure of useful

energy in most processes is the number of ATP

molecules gained or consumed By this measure alone

the yield from both glycolysis and the citric acid

cycle is quite low After two complete turns of the

cycle one molecule of glucose has been completely

oxidized to six molecules of CO2 but only four

molecules of ATP have been produced (a net of two

ATP from glycolysis plus one for each turn of the

cycle) At this point most of the energy associated

with the glucose molecule has been conserved in the

17

form of electron pairs generated by the oxidation of

glycolytic and citric acid cycle intermediates

In this section we will discuss the third stage

of cellular respirationmdashthe transfer of electrons

fromNADH and FADH2 to oxygen and the accompanying

conversion of redox energy to ATP The transfer of

electrons fromNADHand FADH2 to oxygen involves a

sequence of electron carriers arranged in an

electron transport chainMembrane fractionation

studies have shown that the enzymes and electron

carriers making up the electron transport chain are

organized predominantly into four large

multimolecular complexes (complexes ImdashIV) and two

mobile carriers located in the inner mitochondrial

membrane (Figure 109) In this sense there are a

great number of similarities between the

mitochondrial inner membrane and the thylakoid

membranes of the chloroplast (compare Figure 109

with Figure 76) This is not unexpected since the

principal function of each membrane is energy

transformation and many of the same or similar

components are involved

The Components of the Electron Transport Chain

The electron transport chain of the

mitochondria is the means by which electrons are

18

removed from the reduced carrier NADH and

transferred to oxygen to yield H2O

1) NADH

NADH is generated in the matrix by the reactions

of pyruvate dehydrogenase isocitrate

dehydrogenase α-ketoglutarate dehydrogenase and

malate dehyrogenase The electron transport chain

begins with reoxidizing NADH to form NAD+ and

channeling the electrons into the formation of

reduced coenzymes Important to note that NADH

transfers 2 electrons at a time in the form of a

hydride

NAD+ + 2e- + H+ 1048581 NADH Eorsquo = minus0315 V

2) Flavoproteins

Flavoproteins have either a FAD (flavin adenosine

dinucleotide) or a FMN (flavin mononucleotide)

prosthetic group Flavoproteins can accept or

donate electrons one at time or two at a time

Thus they are often intermediaries between two

electron acceptorsdonors and one electron

acceptorsdonors For flavoproteins the typical

standard reduction potentials are around 0 V

FAD + 2e- + 2H+ 1048581 FADH2 FMN + 2e- + 2H+ 1048581 FMNH2

Eorsquo asymp 0 V

19

3) Coenzyme Q (CoQ)

aka ubiquinone (UQ) Shown below CoQ has ten

repeating isoprene units which make it

insoluble in water but soluble in the

hydrophobic lipid bylayer Coenzyme Q is a

versatile cofactor because it is a soluble

electron carrier in the hydrophobic bilipid layer

of the

inner mitochondrial membrane Like flavoproteins

CoQ can acceptdonate electrons one at a time or

two at a time

Q + 2e- + 2H+ 1048581 QH2 Eorsquo =0060 V

Q + e- + H+ 1048581 1048581QH Eorsquo =0030 V

1048581QH + e- + H+ 1048581 QH2 Eorsquo =0190 V

4) Cytochromes

Cytochromes are proteins that contain heme

prosthetic groups which function as

one electron carriers The heme iron is involved

in one electron transfers involving the Fe2+ and

Fe3+ oxidation states Cytochromes are named by

their absorption spectra which depends

on the porphyrin structure and environment The

example shown is the heme prosthetic group of

cytochrome b Cytochrome b contains the same iron

porphyrin found in hemoglobin and myoglobin

20

Other cytochromes we will encounter in the

electron transport complexes are cytochromes b

c c1 a and a3

5) Iron-Sulfur Proteins

In the electron transport chain we will encounter

many iron-sulfer proteins which participate in

one electron transfers involving the the Fe2+ and

Fe3+ oxidation states These are non-heme iron-

sulfur proteinsThe simplest iron-sulfer protein

is FeS in which iron is tetrahedrally coordinated

by four cysteines The second form is Fe2S2 which

contains two irons complexed to 2 cysteine

residues and two inorganic sulfides The third

form is Fe3S4 which contains 3 iron atoms

coordinated to three cysteine residues and 4

inorganic sulfides The last form is the most

complicated Fe4S4 which contains 4 iron atoms

coordinated to 4 cysteine residues and 4

inorganic sulfides

6) Copper Proteins

Copper bound proteins participate in one electron

transfers involving the Cu+ and Cu2+ oxidation

states

21

Overview of the Electron Transport Chain

Electrons move along the electron transport

chain going from donor to acceptor until they reach

oxygen the ultimate electron acceptor The standard

reduction potentials of the electron carriers are

between the NADHNAD+ couple (-0315 V) and the

oxygenH2O couple (0816 V) as on the next page The

components of the electron transport chain are

organized into 4 complexes Each complex contains

several different electron carriers

1 Complex I also known as the NADH-coenzyme Q

reductase or NADH dehydrogenase

2 Complex II also known as succinate-coenzyme Q

reductase or succinate dehydrogenase

3 Complex III also known as coenzyme Q reductase

4 Complex IV also known as cytochrome c reductase

The free energy necessary to generate ATP

is extracted from the oxidation of NADH and

FADH2 by the electron transport chain It

consists of four protein complexes and single

protein chain of cytochrome c through which

electrons pass from lower to higher standard

redox potentials Electrons are carried from complex

I and II to complex III by the membrane (lipid)

soluble coenzyme Q (CoQ or ubiquinone) and between

22

complexes III and IV by the peripheral membrane

protein cytochrome c

In the following I give the overall

reactions catalysed by corresponding protein

complexes in the mitochondrial electron transport

chain

1 Complex I

Complex I is also called NADH-Coenzyme Q

reductase because this large protein complex

transfers 2 electrons from NADH to coenzyme Q

Complex I was formerly known as NADH dehydrogenase

Complex I is huge 850000 kD and is composed of more

than thirty subunits It contains a FMN prosthetic

group and seven or more Fe-S clusters This complex

has between 20-26 iron atoms boundThe prosthetic

group FMN is absolutely required for activity

Therefore this complex is a flavoprotein This

complex binds NADH transfers two electrons in the

form of a hydride to FMN to produce NAD+ and FMNH2

23

The subsequent steps involve the transfer of

electrons one at a time to a series of iron-sulfer

complexes that includes both 2Fe-2S and 4Fe-4S

clusters Note the importance of FMN First it

functions as a 2 electron acceptor in the hydride

transfer from NADH Second it functions as a 1

electron donor to the series of iron sulfur clusters

FMN and FAD often play crucial links between 2

electron transfer agents and 1 electron transfer

agents The final step of this complex is the

transfer of 2 electrons one at a time to coenzyme Q

CoQ like FMN and FAD can function as a 2 electron

donoracceptor and as a 1 electron donoracceptor

CoQ is a mobile electron carrier because its

isoprenoid tail makes it highly hydrophobic and

lipophillic It diffuses freely in the bilipid layer

of the inner mitochondrial membrane

The process of transferring electrons from NADH

to CoQ by complex I results in the net transport of

protons from the matrix side of the inner

mitochondrial membrane to the inter membrane space

where the H+ ions accumulate generating a proton

motive force The intermembrane space side of the

inner membrane is referred to as the P face (P

standing for positive) The matrix side of the inner

membrane is referred to as the the N face The

24

transport of electrons from NADH to CoQ is coupled to

the transport of protons across the membrane This is

an example of active transport The stiochiometry is

4 H+ transported per 2 electrons

2 Complex II

Believe it or not you are already familiar

with Complex II It is none other than succinate

dehydrogenase The only enzyme of the citric acid

cycle that is an integral membrane protein This

complex is composed of four subunits 2 of which are

iron-sulfur proteins and the other two subunits

together bind FAD through a covalent link to a

histidine residue These two subunits are called

flavoprotein 2 or FP2 Complex II contains 3 Fe-S

centers 1 4Fe-4S cluster 1 3Fe-4S cluster and 1

2Fe- 2S cluster In the first step of this complex

succinate is bound and a hydride is transferred to

FAD to generate FADH2 and fumarate FADH2 then

transfers its electrons one at a time to the Fe-S

centers Thus once again FAD functions as 2 electron

acceptor and a 1 electron donor The final step of

this complex is the transfer of 2 electrons one at a

time to coenzyme Q to produce CoQH2 For complex II

the standard free energy change of the overall

reaction is too small to drive the transport of

protons across the inner mitochondrial membrane This

25

accounts for the 15 ATPrsquos generated per FADH2

compared with the 25 ATPrsquos generated per NADH

3 Complex III

This complex is also known as coenzyme Q-

cytochrome c reductase because it passes the

electrons form CoQH2 to cyt c through a very unique

electron transport pathway called the Q-cycle Shown

to the left are the porphyrins found in cytochromes

Cytochrome b contains the same iron protoporphyrin as

hemoglobin and myoglobin The c cytochromes contain

heme c through covalent attachment by cysteine

residues Cytochrome a is found in two forms in

complex IV In complex III we find two b-type

cytochromes and one c-type cytochrome Complex III is

complex and we have a crystal structure

4 Complex IV

Complex IV is also known as cytochrome c oxidase

because it accepts the electrons from cytochrome c

and directs them towards the four electron reduction

of O2 to form 2 molecules of H2O

4 cyt c (Fe2+) + 4 H+ + O2 4 cyt c (Fe3+) +

2H2O

26

Cytochrome c oxidase contains 2 heme

centerscytochrome a and cytochrome a3 and two copper

proteins Each of the protein bound coppers are

associated with one of the cytochromes The copper

sites are called CuA and CuB CuA is associated with

cytochrome a and is shown to the left CuB is

associated with cytochrome a3 The copper sites

function as 1 electron carriers cycling between the

cuprous state Cu+ and the cupric state Cu2+ Just

like iron containing proteins they transfer

electrons one at a time Cytochrome c is bound on the

P-face of the membrane and transfers its electron to

CuA The oxidized cytochrome c dissociates CuA then

transfers the electron to cytochrome a The protein

bound CuA and the iron bound in cytochrome a are 15 Aring

apart In contrast the CuB and the iron bound in

cytochrome a3 are very close to each other forming a

binuclear metal center shown below

27

Electron transport chains are associated with

membranes such as the mitochondrial membrane in

eukaryotic organisms the thylakoid membrane of

photosynthetic organisms and the plasma membrane of

prokaryotic organisms While some of the details

differ electron transport chains in all organisms

operate on the same basic principles

Excited electrons are brought to the electron

transport chain by electron carriers such as NADH

and FADH2 In photosynthetic organisms the electrons

are donated by water and excited by light energy

The electron donor is oxidized as the electrons are

passed to the first protein complex in the chain As

it accepts the electrons the protein complex is

reduced Next the electrons are passed from the

28

first protein complex which becomes oxidized to

the second protein complex which becomes reduced

Each of these redox reactions is slightly exergonic

or energy releasing Some of this energy is used to

move protons across the membrane against their

concentration gradient With each successive

transfer of electrons the original excited

electrons lose some of their energy After several

successive redox reactions the electrons are

donated to a terminal electron acceptor In aerobic

respiration the terminal electron acceptor is

oxygen In photosynthesis the terminal electron

acceptor is a compound called NADP

The energy in the excited electrons has been

transformed into an electrochemical gradient

represented by the high concentration of protons on

one side of the membrane These protons can diffuse

back across the membrane by passing through a

protein complex called ATP synthase As these

protons pass through ATP synthase down their

concentration gradient they provide the energy

needed to make ATP from ADP and inorganic phosphate

within the cell

29

CHAPTER III

CLOSING

A Conclution

When a hexose is completely oxidized to CO2

and H2O using these three processes Glycolysis

yields two ATP and two NADH per hexose used Each

such NADH oxidized by the electron transport system

30

yields two ATP as dscribed above so glycolysis

contributes a total of six ATP per hexose The Krebs

cycle contributes two ATP per hexose or per two

pyruvates when succinyl CoA is cleaved to succinate

and CoASH This cycle also produces eight NADH per

hexose within the mitochondrial matrix by oxidative

phosporylation each of these NADH yields three ATP

or 24 per hexose Each FADH2 from the Krebs cycle

yields two ATP by oxydative phosporylation or four

per hexose (two pyruvates) The total contribution of

the Krebs cycle is then 30 Adding these 30 to the 6

from glycolysis leads to a total of 36 per hexose

completely respired by these process

B Suggestion

My greatest wish is that the readers find

this manuscript helpful in gaining the knowledge they

need for effective learning and practice and in

preparing for examination designed to test that

knowledge Reader comments and suggestions are

welcome and would be very helpful in improving our

manuscript Your contribution may assist many other

students in the coming years

31

REFERENCE

Anonim 2013 Anaerobic Respiration

httpwwwscoolcoukgcsebiologyrespiration

revise-itanaerobic-respiration Accessed

September 10th 2013

Anonim 2013 What is the difference between aerobic

and anaerobic respiration in plants

httpwwwbiologylifeeasyorg4010difference

-between-aerobic-anaerobic-respiration-plants

Accessed September 10th 2013

Campbell Mary K 2006 Biochemistry New York Thomson

Brooks Cole Inc

Dat J Capelli N Folzer H Bourgeade P Badot P-M

(2004) Sensing and signaling during

plantflooding Plant Physiology and

Biochemistry 42 273-282

Dat J Folzer H Parent C Badot P-M Capelli N

(2006) HypoxiastressCurrent Understanding and

Perspectives In Teixeira da Silva JA (Ed)

Floriculture Ornamental and Plant

Biotechnology Advances and Topical Issues (Vol

3) Global Science Books Isleworth United

Kingdompp 664-674

Oxalosucciante is changed into α-ketoglutrate by

iso-citrate dehydrogenase with the help of

coenzyme NAD or NADP In this reaction carbon

dioxide and NADH2 or NADPH2 are also released

7 Formation of succinyl CoA

α-ketoglutarate combines with acetyl CoA in the

presence of coenzyme NAD and enzyme α-

keoglutaratedehyrogenase to form succinyl coenzyme

a carbon dioxide and NADH2

8 Formation of Succinate

Later on coenzyme a is removed from succinyl

coenzyme A in the presence of guanosinediphosphate

(GDP) and inorganic phosphate to form succinate

and guanosine triphosphate (GTP) This reaction is

carried out by an enzyme called succinyl CoA

synthatase

9 Formation of Funarate

An enzyme succinic dehydrogenase removes hydrogen

from succinate to form funarate

10 Formation of Malate

Fumarate reacts with water in the presence of

enzyme fumarase to form malate

11 Regeneration of oxaloacetate

Malate is oxidized by malic dehydrogenase and NAD

forming oxaloacetate and NADH2 Thus oxalo acetate

is again available to start another cycle

9

(httpkrebs-cycle-or-citric-acid-cycle-or-

trihtml)

Importance of CTA Cycle

1 Source of energy

In addition to routine organic compounds described

above by products like nicotinanide adenine

dinucleotide (NADH2) and guanosine triphosphate

(GTP) are the source of biological energy NADH2

after oxidation produce energy whereas GTP is

itself high energy phosphate compound

2 Oxidation of organic compounds taken as food

Oxidation of fats carbohydrates and proteins take

place through it or in other words it can be said

that oxidation of all compounds having carbon

atoms can take place through TCA cycle Some of

amino acids like alanine glutamic acid and

aspartic acid at one stage or the other enters

into TCA cycle eg glutamic acid enters cycle

after its transformation into α-ketoglutarate

Similarly alanine enters the cycle after its

conversion into pyruvate

3 Intermediate compounds

TCA cycle is also involved in synthesis of

intermediate compounds leading to the formation of

larger molecules (Ray 1963 17)

10

Verification of Krebs Cycle

It was done by radioactive traces like C14 as

radioactive carbon dioxide of different levels and

reactions After addition of radioactive carbon

dioxide different chemical compounds produced like

glucose fats amino acids were isolated and looked

for radioactive carbon In this way whole of

metabolic reaction were verified including individual

reactions alternative metabolic pathways

intermediates of fats carbohydrates and amino acids

etc in the body cells as well as in test tubes

(Wilson 1962 99)

The primary function of the Krebs cycle are as

follows

1 Reduction of NAD+ and FAD to the electron donors

NADH and FADH2 that are subsequently oxidized to

yield ATP

2 Direct synthesis of a limited amount of ATP (1 ATP

for each pyruvate oxidized)

3 Formation of carbon skeletons that can be used to

synthesize certain amino acids that in turn are

converted into larger molecules

11

B Fermentation

Although glycolysis can function well without

O2 the further oxidation of pyruvate and NADH by

mitochondria requires this gas Thus when O2 is

limiting NADH and pyruvate begin to accumulate

Under this condition plants carry out fermentation

(anaerobic respiration) forming either ethanol or

lactic acid (usually ethanol) As shown in the

picture below the two top reactions consists of a

decarboxylation to form acetaldehide then rapid

reduction of acetaldehide by NADH to form ethanol

These reactions are catalyzed by alcohol

dehydrogenase Some cells contain lactic acid

12

dehydrogenase which uses NADH to reduce pyruvic to

lactic acid Thus either ethanol or lactic acid or

both are fermentation products depending on the

activities of each dehydrogenase present In each

case NADH is the reductant and only under

anaerobic conditions is it abundant anough to cause

reduction Furthermore in some plants NADH is used

to cause accumulation of the other compounds when O2

is limiting especially malate and glycerol

(Salisbury 1969 233)

The fate of pyruvate produced by glycolysis

depends primarily on whether or not oxygen is

present Under normal aerobic conditions pyruvate

is transported into the mitochondrion where it is

further oxidized to CO2 and water transferring its

electrons ultimately to molecular oxygen

13

Although higher plants are obligate aerobs and

are able to tolerate anoxia for only short periods

tissues or organs are occasionally subjected to

anaerobic conditions A typical siuation is that of

roots when the soil is saturated with water When

there is no oxygen to serve as the terminal electron

acceptor mitochondrial respiration will shut down

and metabolism will shift over to fermentation

Fermentation converts pyruvate to either ethanol

through the action of the enzyme alcohol

dehydrogenase (ADH) or lactate via lactate

dehydrogenase (LDH) In most plants the principal

products of fermentation are CO2 and ethanol Some

lactate may be formed primarily in the early stages

of anoxia However lactate lowers he pH of the

cytosol which in turn activates pyruvate

decarboxylase and initiates the production of

ethanol

Either one of the fermentation reactions

consumes the NADH produced earlier in glycolysis by

the oxidation of glyceraldehide-3-P Although this

means there is no net gain of reducing potential in

fermentation This recycling of NADH is still

important to the cell The poolof NADH plus NAD+ in

the cell is relatively small and if the NADH is not

recycled there will be no supply of NAD+ to support

14

the continued oxidation of glyceraldehyde-3-P if

this were the case glycolysis and the producton of

even the small quantities of ATP necessary to

maintain the cells under anaerobic conditions would

then grind to a halt (Hopkins 2009 241)

Soil waterlogging has long been identified as a

major abiotic stress and the constraints that it

provides the roots have a significant effect on the

growth and development of plants When this event

occurs in the spring then these puddles can reduce

seed germination and seedling development Thus a

pool of water is an important factor that affects

the growth development and survival of plant

species not only in natural ecosystems but also

the agricultural and horticultural systems (Dat et

al 2006)

After flooding rapid changes in soil

properties At the time of pore water meets the

land the air pushed out gas diffusion is reduced

and toxic compounds accumulate due to anaerobic

conditions All of these changes greatly affect the

ability of plants to survive In response increased

stomatal resistance photosynthesis and root

hydraulic conductivity decreases and reduced

translocation fotoassimilat Nevertheless one of

15

the best adaptations of plants to hypoxia anoxia

is transitional biochemical and metabolic processes

common to current limited availability of O2 (Dat et

al 2004)

Selective synthesis of a set of about 20

anaerobic stress proteins (ANPS) allows the process

of producing energy without oxygen metabolism under

anaerobic conditions (Subbaiah and Sachs 2003)

Another adaptation is the observed morphological

changes consisting of the formation of hypertrophic

lenticels adventitious root initiation and or

progression of aerenchyma (Vartapetian and Jackson

1997 Jackson and Colmer 2005 Folzer et al 2006)

This review details the diverse plant stress

response to hypoxia anoxia caused by soil

waterlogging flooding and examines some of the key

features of metabolic adaptation physiological and

morphological

16

C Electron

Transport System

We noted earlier that one of the principal

functions of the respiration is to retrieve in

useful form some of the energy initially stored in

assimilates Our traditional measure of useful

energy in most processes is the number of ATP

molecules gained or consumed By this measure alone

the yield from both glycolysis and the citric acid

cycle is quite low After two complete turns of the

cycle one molecule of glucose has been completely

oxidized to six molecules of CO2 but only four

molecules of ATP have been produced (a net of two

ATP from glycolysis plus one for each turn of the

cycle) At this point most of the energy associated

with the glucose molecule has been conserved in the

17

form of electron pairs generated by the oxidation of

glycolytic and citric acid cycle intermediates

In this section we will discuss the third stage

of cellular respirationmdashthe transfer of electrons

fromNADH and FADH2 to oxygen and the accompanying

conversion of redox energy to ATP The transfer of

electrons fromNADHand FADH2 to oxygen involves a

sequence of electron carriers arranged in an

electron transport chainMembrane fractionation

studies have shown that the enzymes and electron

carriers making up the electron transport chain are

organized predominantly into four large

multimolecular complexes (complexes ImdashIV) and two

mobile carriers located in the inner mitochondrial

membrane (Figure 109) In this sense there are a

great number of similarities between the

mitochondrial inner membrane and the thylakoid

membranes of the chloroplast (compare Figure 109

with Figure 76) This is not unexpected since the

principal function of each membrane is energy

transformation and many of the same or similar

components are involved

The Components of the Electron Transport Chain

The electron transport chain of the

mitochondria is the means by which electrons are

18

removed from the reduced carrier NADH and

transferred to oxygen to yield H2O

1) NADH

NADH is generated in the matrix by the reactions

of pyruvate dehydrogenase isocitrate

dehydrogenase α-ketoglutarate dehydrogenase and

malate dehyrogenase The electron transport chain

begins with reoxidizing NADH to form NAD+ and

channeling the electrons into the formation of

reduced coenzymes Important to note that NADH

transfers 2 electrons at a time in the form of a

hydride

NAD+ + 2e- + H+ 1048581 NADH Eorsquo = minus0315 V

2) Flavoproteins

Flavoproteins have either a FAD (flavin adenosine

dinucleotide) or a FMN (flavin mononucleotide)

prosthetic group Flavoproteins can accept or

donate electrons one at time or two at a time

Thus they are often intermediaries between two

electron acceptorsdonors and one electron

acceptorsdonors For flavoproteins the typical

standard reduction potentials are around 0 V

FAD + 2e- + 2H+ 1048581 FADH2 FMN + 2e- + 2H+ 1048581 FMNH2

Eorsquo asymp 0 V

19

3) Coenzyme Q (CoQ)

aka ubiquinone (UQ) Shown below CoQ has ten

repeating isoprene units which make it

insoluble in water but soluble in the

hydrophobic lipid bylayer Coenzyme Q is a

versatile cofactor because it is a soluble

electron carrier in the hydrophobic bilipid layer

of the

inner mitochondrial membrane Like flavoproteins

CoQ can acceptdonate electrons one at a time or

two at a time

Q + 2e- + 2H+ 1048581 QH2 Eorsquo =0060 V

Q + e- + H+ 1048581 1048581QH Eorsquo =0030 V

1048581QH + e- + H+ 1048581 QH2 Eorsquo =0190 V

4) Cytochromes

Cytochromes are proteins that contain heme

prosthetic groups which function as

one electron carriers The heme iron is involved

in one electron transfers involving the Fe2+ and

Fe3+ oxidation states Cytochromes are named by

their absorption spectra which depends

on the porphyrin structure and environment The

example shown is the heme prosthetic group of

cytochrome b Cytochrome b contains the same iron

porphyrin found in hemoglobin and myoglobin

20

Other cytochromes we will encounter in the

electron transport complexes are cytochromes b

c c1 a and a3

5) Iron-Sulfur Proteins

In the electron transport chain we will encounter

many iron-sulfer proteins which participate in

one electron transfers involving the the Fe2+ and

Fe3+ oxidation states These are non-heme iron-

sulfur proteinsThe simplest iron-sulfer protein

is FeS in which iron is tetrahedrally coordinated

by four cysteines The second form is Fe2S2 which

contains two irons complexed to 2 cysteine

residues and two inorganic sulfides The third

form is Fe3S4 which contains 3 iron atoms

coordinated to three cysteine residues and 4

inorganic sulfides The last form is the most

complicated Fe4S4 which contains 4 iron atoms

coordinated to 4 cysteine residues and 4

inorganic sulfides

6) Copper Proteins

Copper bound proteins participate in one electron

transfers involving the Cu+ and Cu2+ oxidation

states

21

Overview of the Electron Transport Chain

Electrons move along the electron transport

chain going from donor to acceptor until they reach

oxygen the ultimate electron acceptor The standard

reduction potentials of the electron carriers are

between the NADHNAD+ couple (-0315 V) and the

oxygenH2O couple (0816 V) as on the next page The

components of the electron transport chain are

organized into 4 complexes Each complex contains

several different electron carriers

1 Complex I also known as the NADH-coenzyme Q

reductase or NADH dehydrogenase

2 Complex II also known as succinate-coenzyme Q

reductase or succinate dehydrogenase

3 Complex III also known as coenzyme Q reductase

4 Complex IV also known as cytochrome c reductase

The free energy necessary to generate ATP

is extracted from the oxidation of NADH and

FADH2 by the electron transport chain It

consists of four protein complexes and single

protein chain of cytochrome c through which

electrons pass from lower to higher standard

redox potentials Electrons are carried from complex

I and II to complex III by the membrane (lipid)

soluble coenzyme Q (CoQ or ubiquinone) and between

22

complexes III and IV by the peripheral membrane

protein cytochrome c

In the following I give the overall

reactions catalysed by corresponding protein

complexes in the mitochondrial electron transport

chain

1 Complex I

Complex I is also called NADH-Coenzyme Q

reductase because this large protein complex

transfers 2 electrons from NADH to coenzyme Q

Complex I was formerly known as NADH dehydrogenase

Complex I is huge 850000 kD and is composed of more

than thirty subunits It contains a FMN prosthetic

group and seven or more Fe-S clusters This complex

has between 20-26 iron atoms boundThe prosthetic

group FMN is absolutely required for activity

Therefore this complex is a flavoprotein This

complex binds NADH transfers two electrons in the

form of a hydride to FMN to produce NAD+ and FMNH2

23

The subsequent steps involve the transfer of

electrons one at a time to a series of iron-sulfer

complexes that includes both 2Fe-2S and 4Fe-4S

clusters Note the importance of FMN First it

functions as a 2 electron acceptor in the hydride

transfer from NADH Second it functions as a 1

electron donor to the series of iron sulfur clusters

FMN and FAD often play crucial links between 2

electron transfer agents and 1 electron transfer

agents The final step of this complex is the

transfer of 2 electrons one at a time to coenzyme Q

CoQ like FMN and FAD can function as a 2 electron

donoracceptor and as a 1 electron donoracceptor

CoQ is a mobile electron carrier because its

isoprenoid tail makes it highly hydrophobic and

lipophillic It diffuses freely in the bilipid layer

of the inner mitochondrial membrane

The process of transferring electrons from NADH

to CoQ by complex I results in the net transport of

protons from the matrix side of the inner

mitochondrial membrane to the inter membrane space

where the H+ ions accumulate generating a proton

motive force The intermembrane space side of the

inner membrane is referred to as the P face (P

standing for positive) The matrix side of the inner

membrane is referred to as the the N face The

24

transport of electrons from NADH to CoQ is coupled to

the transport of protons across the membrane This is

an example of active transport The stiochiometry is

4 H+ transported per 2 electrons

2 Complex II

Believe it or not you are already familiar

with Complex II It is none other than succinate

dehydrogenase The only enzyme of the citric acid

cycle that is an integral membrane protein This

complex is composed of four subunits 2 of which are

iron-sulfur proteins and the other two subunits

together bind FAD through a covalent link to a

histidine residue These two subunits are called

flavoprotein 2 or FP2 Complex II contains 3 Fe-S

centers 1 4Fe-4S cluster 1 3Fe-4S cluster and 1

2Fe- 2S cluster In the first step of this complex

succinate is bound and a hydride is transferred to

FAD to generate FADH2 and fumarate FADH2 then

transfers its electrons one at a time to the Fe-S

centers Thus once again FAD functions as 2 electron

acceptor and a 1 electron donor The final step of

this complex is the transfer of 2 electrons one at a

time to coenzyme Q to produce CoQH2 For complex II

the standard free energy change of the overall

reaction is too small to drive the transport of

protons across the inner mitochondrial membrane This

25

accounts for the 15 ATPrsquos generated per FADH2

compared with the 25 ATPrsquos generated per NADH

3 Complex III

This complex is also known as coenzyme Q-

cytochrome c reductase because it passes the

electrons form CoQH2 to cyt c through a very unique

electron transport pathway called the Q-cycle Shown

to the left are the porphyrins found in cytochromes

Cytochrome b contains the same iron protoporphyrin as

hemoglobin and myoglobin The c cytochromes contain

heme c through covalent attachment by cysteine

residues Cytochrome a is found in two forms in

complex IV In complex III we find two b-type

cytochromes and one c-type cytochrome Complex III is

complex and we have a crystal structure

4 Complex IV

Complex IV is also known as cytochrome c oxidase

because it accepts the electrons from cytochrome c

and directs them towards the four electron reduction

of O2 to form 2 molecules of H2O

4 cyt c (Fe2+) + 4 H+ + O2 4 cyt c (Fe3+) +

2H2O

26

Cytochrome c oxidase contains 2 heme

centerscytochrome a and cytochrome a3 and two copper

proteins Each of the protein bound coppers are

associated with one of the cytochromes The copper

sites are called CuA and CuB CuA is associated with

cytochrome a and is shown to the left CuB is

associated with cytochrome a3 The copper sites

function as 1 electron carriers cycling between the

cuprous state Cu+ and the cupric state Cu2+ Just

like iron containing proteins they transfer

electrons one at a time Cytochrome c is bound on the

P-face of the membrane and transfers its electron to

CuA The oxidized cytochrome c dissociates CuA then

transfers the electron to cytochrome a The protein

bound CuA and the iron bound in cytochrome a are 15 Aring

apart In contrast the CuB and the iron bound in

cytochrome a3 are very close to each other forming a

binuclear metal center shown below

27

Electron transport chains are associated with

membranes such as the mitochondrial membrane in

eukaryotic organisms the thylakoid membrane of

photosynthetic organisms and the plasma membrane of

prokaryotic organisms While some of the details

differ electron transport chains in all organisms

operate on the same basic principles

Excited electrons are brought to the electron

transport chain by electron carriers such as NADH

and FADH2 In photosynthetic organisms the electrons

are donated by water and excited by light energy

The electron donor is oxidized as the electrons are

passed to the first protein complex in the chain As

it accepts the electrons the protein complex is

reduced Next the electrons are passed from the

28

first protein complex which becomes oxidized to

the second protein complex which becomes reduced

Each of these redox reactions is slightly exergonic

or energy releasing Some of this energy is used to

move protons across the membrane against their

concentration gradient With each successive

transfer of electrons the original excited

electrons lose some of their energy After several

successive redox reactions the electrons are

donated to a terminal electron acceptor In aerobic

respiration the terminal electron acceptor is

oxygen In photosynthesis the terminal electron

acceptor is a compound called NADP

The energy in the excited electrons has been

transformed into an electrochemical gradient

represented by the high concentration of protons on

one side of the membrane These protons can diffuse

back across the membrane by passing through a

protein complex called ATP synthase As these

protons pass through ATP synthase down their

concentration gradient they provide the energy

needed to make ATP from ADP and inorganic phosphate

within the cell

29

CHAPTER III

CLOSING

A Conclution

When a hexose is completely oxidized to CO2

and H2O using these three processes Glycolysis

yields two ATP and two NADH per hexose used Each

such NADH oxidized by the electron transport system

30

yields two ATP as dscribed above so glycolysis

contributes a total of six ATP per hexose The Krebs

cycle contributes two ATP per hexose or per two

pyruvates when succinyl CoA is cleaved to succinate

and CoASH This cycle also produces eight NADH per

hexose within the mitochondrial matrix by oxidative

phosporylation each of these NADH yields three ATP

or 24 per hexose Each FADH2 from the Krebs cycle

yields two ATP by oxydative phosporylation or four

per hexose (two pyruvates) The total contribution of

the Krebs cycle is then 30 Adding these 30 to the 6

from glycolysis leads to a total of 36 per hexose

completely respired by these process

B Suggestion

My greatest wish is that the readers find

this manuscript helpful in gaining the knowledge they

need for effective learning and practice and in

preparing for examination designed to test that

knowledge Reader comments and suggestions are

welcome and would be very helpful in improving our

manuscript Your contribution may assist many other

students in the coming years

31

REFERENCE

Anonim 2013 Anaerobic Respiration

httpwwwscoolcoukgcsebiologyrespiration

revise-itanaerobic-respiration Accessed

September 10th 2013

Anonim 2013 What is the difference between aerobic

and anaerobic respiration in plants

httpwwwbiologylifeeasyorg4010difference

-between-aerobic-anaerobic-respiration-plants

Accessed September 10th 2013

Campbell Mary K 2006 Biochemistry New York Thomson

Brooks Cole Inc

Dat J Capelli N Folzer H Bourgeade P Badot P-M

(2004) Sensing and signaling during

plantflooding Plant Physiology and

Biochemistry 42 273-282

Dat J Folzer H Parent C Badot P-M Capelli N

(2006) HypoxiastressCurrent Understanding and

Perspectives In Teixeira da Silva JA (Ed)

Floriculture Ornamental and Plant

Biotechnology Advances and Topical Issues (Vol

3) Global Science Books Isleworth United

Kingdompp 664-674

(httpkrebs-cycle-or-citric-acid-cycle-or-

trihtml)

Importance of CTA Cycle

1 Source of energy

In addition to routine organic compounds described

above by products like nicotinanide adenine

dinucleotide (NADH2) and guanosine triphosphate

(GTP) are the source of biological energy NADH2

after oxidation produce energy whereas GTP is

itself high energy phosphate compound

2 Oxidation of organic compounds taken as food

Oxidation of fats carbohydrates and proteins take

place through it or in other words it can be said

that oxidation of all compounds having carbon

atoms can take place through TCA cycle Some of

amino acids like alanine glutamic acid and

aspartic acid at one stage or the other enters

into TCA cycle eg glutamic acid enters cycle

after its transformation into α-ketoglutarate

Similarly alanine enters the cycle after its

conversion into pyruvate

3 Intermediate compounds

TCA cycle is also involved in synthesis of

intermediate compounds leading to the formation of

larger molecules (Ray 1963 17)

10

Verification of Krebs Cycle

It was done by radioactive traces like C14 as

radioactive carbon dioxide of different levels and

reactions After addition of radioactive carbon

dioxide different chemical compounds produced like

glucose fats amino acids were isolated and looked

for radioactive carbon In this way whole of

metabolic reaction were verified including individual

reactions alternative metabolic pathways

intermediates of fats carbohydrates and amino acids

etc in the body cells as well as in test tubes

(Wilson 1962 99)

The primary function of the Krebs cycle are as

follows

1 Reduction of NAD+ and FAD to the electron donors

NADH and FADH2 that are subsequently oxidized to

yield ATP

2 Direct synthesis of a limited amount of ATP (1 ATP

for each pyruvate oxidized)

3 Formation of carbon skeletons that can be used to

synthesize certain amino acids that in turn are

converted into larger molecules

11

B Fermentation

Although glycolysis can function well without

O2 the further oxidation of pyruvate and NADH by

mitochondria requires this gas Thus when O2 is

limiting NADH and pyruvate begin to accumulate

Under this condition plants carry out fermentation

(anaerobic respiration) forming either ethanol or

lactic acid (usually ethanol) As shown in the

picture below the two top reactions consists of a

decarboxylation to form acetaldehide then rapid

reduction of acetaldehide by NADH to form ethanol

These reactions are catalyzed by alcohol

dehydrogenase Some cells contain lactic acid

12

dehydrogenase which uses NADH to reduce pyruvic to

lactic acid Thus either ethanol or lactic acid or

both are fermentation products depending on the

activities of each dehydrogenase present In each

case NADH is the reductant and only under

anaerobic conditions is it abundant anough to cause

reduction Furthermore in some plants NADH is used

to cause accumulation of the other compounds when O2

is limiting especially malate and glycerol

(Salisbury 1969 233)

The fate of pyruvate produced by glycolysis

depends primarily on whether or not oxygen is

present Under normal aerobic conditions pyruvate

is transported into the mitochondrion where it is

further oxidized to CO2 and water transferring its

electrons ultimately to molecular oxygen

13

Although higher plants are obligate aerobs and

are able to tolerate anoxia for only short periods

tissues or organs are occasionally subjected to

anaerobic conditions A typical siuation is that of

roots when the soil is saturated with water When

there is no oxygen to serve as the terminal electron

acceptor mitochondrial respiration will shut down

and metabolism will shift over to fermentation

Fermentation converts pyruvate to either ethanol

through the action of the enzyme alcohol

dehydrogenase (ADH) or lactate via lactate

dehydrogenase (LDH) In most plants the principal

products of fermentation are CO2 and ethanol Some

lactate may be formed primarily in the early stages

of anoxia However lactate lowers he pH of the

cytosol which in turn activates pyruvate

decarboxylase and initiates the production of

ethanol

Either one of the fermentation reactions

consumes the NADH produced earlier in glycolysis by

the oxidation of glyceraldehide-3-P Although this

means there is no net gain of reducing potential in

fermentation This recycling of NADH is still

important to the cell The poolof NADH plus NAD+ in

the cell is relatively small and if the NADH is not

recycled there will be no supply of NAD+ to support

14

the continued oxidation of glyceraldehyde-3-P if

this were the case glycolysis and the producton of

even the small quantities of ATP necessary to

maintain the cells under anaerobic conditions would

then grind to a halt (Hopkins 2009 241)

Soil waterlogging has long been identified as a

major abiotic stress and the constraints that it

provides the roots have a significant effect on the

growth and development of plants When this event

occurs in the spring then these puddles can reduce

seed germination and seedling development Thus a

pool of water is an important factor that affects

the growth development and survival of plant

species not only in natural ecosystems but also

the agricultural and horticultural systems (Dat et

al 2006)

After flooding rapid changes in soil

properties At the time of pore water meets the

land the air pushed out gas diffusion is reduced

and toxic compounds accumulate due to anaerobic

conditions All of these changes greatly affect the

ability of plants to survive In response increased

stomatal resistance photosynthesis and root

hydraulic conductivity decreases and reduced

translocation fotoassimilat Nevertheless one of

15

the best adaptations of plants to hypoxia anoxia

is transitional biochemical and metabolic processes

common to current limited availability of O2 (Dat et

al 2004)

Selective synthesis of a set of about 20

anaerobic stress proteins (ANPS) allows the process

of producing energy without oxygen metabolism under

anaerobic conditions (Subbaiah and Sachs 2003)

Another adaptation is the observed morphological

changes consisting of the formation of hypertrophic

lenticels adventitious root initiation and or

progression of aerenchyma (Vartapetian and Jackson

1997 Jackson and Colmer 2005 Folzer et al 2006)

This review details the diverse plant stress

response to hypoxia anoxia caused by soil

waterlogging flooding and examines some of the key

features of metabolic adaptation physiological and

morphological

16

C Electron

Transport System

We noted earlier that one of the principal

functions of the respiration is to retrieve in

useful form some of the energy initially stored in

assimilates Our traditional measure of useful

energy in most processes is the number of ATP

molecules gained or consumed By this measure alone

the yield from both glycolysis and the citric acid

cycle is quite low After two complete turns of the

cycle one molecule of glucose has been completely

oxidized to six molecules of CO2 but only four

molecules of ATP have been produced (a net of two

ATP from glycolysis plus one for each turn of the

cycle) At this point most of the energy associated

with the glucose molecule has been conserved in the

17

form of electron pairs generated by the oxidation of

glycolytic and citric acid cycle intermediates

In this section we will discuss the third stage

of cellular respirationmdashthe transfer of electrons

fromNADH and FADH2 to oxygen and the accompanying

conversion of redox energy to ATP The transfer of

electrons fromNADHand FADH2 to oxygen involves a

sequence of electron carriers arranged in an

electron transport chainMembrane fractionation

studies have shown that the enzymes and electron

carriers making up the electron transport chain are

organized predominantly into four large

multimolecular complexes (complexes ImdashIV) and two

mobile carriers located in the inner mitochondrial

membrane (Figure 109) In this sense there are a

great number of similarities between the

mitochondrial inner membrane and the thylakoid

membranes of the chloroplast (compare Figure 109

with Figure 76) This is not unexpected since the

principal function of each membrane is energy

transformation and many of the same or similar

components are involved

The Components of the Electron Transport Chain

The electron transport chain of the

mitochondria is the means by which electrons are

18

removed from the reduced carrier NADH and

transferred to oxygen to yield H2O

1) NADH

NADH is generated in the matrix by the reactions

of pyruvate dehydrogenase isocitrate

dehydrogenase α-ketoglutarate dehydrogenase and

malate dehyrogenase The electron transport chain

begins with reoxidizing NADH to form NAD+ and

channeling the electrons into the formation of

reduced coenzymes Important to note that NADH

transfers 2 electrons at a time in the form of a

hydride

NAD+ + 2e- + H+ 1048581 NADH Eorsquo = minus0315 V

2) Flavoproteins

Flavoproteins have either a FAD (flavin adenosine

dinucleotide) or a FMN (flavin mononucleotide)

prosthetic group Flavoproteins can accept or

donate electrons one at time or two at a time

Thus they are often intermediaries between two

electron acceptorsdonors and one electron

acceptorsdonors For flavoproteins the typical

standard reduction potentials are around 0 V

FAD + 2e- + 2H+ 1048581 FADH2 FMN + 2e- + 2H+ 1048581 FMNH2

Eorsquo asymp 0 V

19

3) Coenzyme Q (CoQ)

aka ubiquinone (UQ) Shown below CoQ has ten

repeating isoprene units which make it

insoluble in water but soluble in the

hydrophobic lipid bylayer Coenzyme Q is a

versatile cofactor because it is a soluble

electron carrier in the hydrophobic bilipid layer

of the

inner mitochondrial membrane Like flavoproteins

CoQ can acceptdonate electrons one at a time or

two at a time

Q + 2e- + 2H+ 1048581 QH2 Eorsquo =0060 V

Q + e- + H+ 1048581 1048581QH Eorsquo =0030 V

1048581QH + e- + H+ 1048581 QH2 Eorsquo =0190 V

4) Cytochromes

Cytochromes are proteins that contain heme

prosthetic groups which function as

one electron carriers The heme iron is involved

in one electron transfers involving the Fe2+ and

Fe3+ oxidation states Cytochromes are named by

their absorption spectra which depends

on the porphyrin structure and environment The

example shown is the heme prosthetic group of

cytochrome b Cytochrome b contains the same iron

porphyrin found in hemoglobin and myoglobin

20

Other cytochromes we will encounter in the

electron transport complexes are cytochromes b

c c1 a and a3

5) Iron-Sulfur Proteins

In the electron transport chain we will encounter

many iron-sulfer proteins which participate in

one electron transfers involving the the Fe2+ and

Fe3+ oxidation states These are non-heme iron-

sulfur proteinsThe simplest iron-sulfer protein

is FeS in which iron is tetrahedrally coordinated

by four cysteines The second form is Fe2S2 which

contains two irons complexed to 2 cysteine

residues and two inorganic sulfides The third

form is Fe3S4 which contains 3 iron atoms

coordinated to three cysteine residues and 4

inorganic sulfides The last form is the most

complicated Fe4S4 which contains 4 iron atoms

coordinated to 4 cysteine residues and 4

inorganic sulfides

6) Copper Proteins

Copper bound proteins participate in one electron

transfers involving the Cu+ and Cu2+ oxidation

states

21

Overview of the Electron Transport Chain

Electrons move along the electron transport

chain going from donor to acceptor until they reach

oxygen the ultimate electron acceptor The standard

reduction potentials of the electron carriers are

between the NADHNAD+ couple (-0315 V) and the

oxygenH2O couple (0816 V) as on the next page The

components of the electron transport chain are

organized into 4 complexes Each complex contains

several different electron carriers

1 Complex I also known as the NADH-coenzyme Q

reductase or NADH dehydrogenase

2 Complex II also known as succinate-coenzyme Q

reductase or succinate dehydrogenase

3 Complex III also known as coenzyme Q reductase

4 Complex IV also known as cytochrome c reductase

The free energy necessary to generate ATP

is extracted from the oxidation of NADH and

FADH2 by the electron transport chain It

consists of four protein complexes and single

protein chain of cytochrome c through which

electrons pass from lower to higher standard

redox potentials Electrons are carried from complex

I and II to complex III by the membrane (lipid)

soluble coenzyme Q (CoQ or ubiquinone) and between

22

complexes III and IV by the peripheral membrane

protein cytochrome c

In the following I give the overall

reactions catalysed by corresponding protein

complexes in the mitochondrial electron transport

chain

1 Complex I

Complex I is also called NADH-Coenzyme Q

reductase because this large protein complex

transfers 2 electrons from NADH to coenzyme Q

Complex I was formerly known as NADH dehydrogenase

Complex I is huge 850000 kD and is composed of more

than thirty subunits It contains a FMN prosthetic

group and seven or more Fe-S clusters This complex

has between 20-26 iron atoms boundThe prosthetic

group FMN is absolutely required for activity

Therefore this complex is a flavoprotein This

complex binds NADH transfers two electrons in the

form of a hydride to FMN to produce NAD+ and FMNH2

23

The subsequent steps involve the transfer of

electrons one at a time to a series of iron-sulfer

complexes that includes both 2Fe-2S and 4Fe-4S

clusters Note the importance of FMN First it

functions as a 2 electron acceptor in the hydride

transfer from NADH Second it functions as a 1

electron donor to the series of iron sulfur clusters

FMN and FAD often play crucial links between 2

electron transfer agents and 1 electron transfer

agents The final step of this complex is the

transfer of 2 electrons one at a time to coenzyme Q

CoQ like FMN and FAD can function as a 2 electron

donoracceptor and as a 1 electron donoracceptor

CoQ is a mobile electron carrier because its

isoprenoid tail makes it highly hydrophobic and

lipophillic It diffuses freely in the bilipid layer

of the inner mitochondrial membrane

The process of transferring electrons from NADH

to CoQ by complex I results in the net transport of

protons from the matrix side of the inner

mitochondrial membrane to the inter membrane space

where the H+ ions accumulate generating a proton

motive force The intermembrane space side of the

inner membrane is referred to as the P face (P

standing for positive) The matrix side of the inner

membrane is referred to as the the N face The

24

transport of electrons from NADH to CoQ is coupled to

the transport of protons across the membrane This is

an example of active transport The stiochiometry is

4 H+ transported per 2 electrons

2 Complex II

Believe it or not you are already familiar

with Complex II It is none other than succinate

dehydrogenase The only enzyme of the citric acid

cycle that is an integral membrane protein This

complex is composed of four subunits 2 of which are

iron-sulfur proteins and the other two subunits

together bind FAD through a covalent link to a

histidine residue These two subunits are called

flavoprotein 2 or FP2 Complex II contains 3 Fe-S

centers 1 4Fe-4S cluster 1 3Fe-4S cluster and 1

2Fe- 2S cluster In the first step of this complex

succinate is bound and a hydride is transferred to

FAD to generate FADH2 and fumarate FADH2 then

transfers its electrons one at a time to the Fe-S

centers Thus once again FAD functions as 2 electron

acceptor and a 1 electron donor The final step of

this complex is the transfer of 2 electrons one at a

time to coenzyme Q to produce CoQH2 For complex II

the standard free energy change of the overall

reaction is too small to drive the transport of

protons across the inner mitochondrial membrane This

25

accounts for the 15 ATPrsquos generated per FADH2

compared with the 25 ATPrsquos generated per NADH

3 Complex III

This complex is also known as coenzyme Q-

cytochrome c reductase because it passes the

electrons form CoQH2 to cyt c through a very unique

electron transport pathway called the Q-cycle Shown

to the left are the porphyrins found in cytochromes

Cytochrome b contains the same iron protoporphyrin as

hemoglobin and myoglobin The c cytochromes contain

heme c through covalent attachment by cysteine

residues Cytochrome a is found in two forms in

complex IV In complex III we find two b-type

cytochromes and one c-type cytochrome Complex III is

complex and we have a crystal structure

4 Complex IV

Complex IV is also known as cytochrome c oxidase

because it accepts the electrons from cytochrome c

and directs them towards the four electron reduction

of O2 to form 2 molecules of H2O

4 cyt c (Fe2+) + 4 H+ + O2 4 cyt c (Fe3+) +

2H2O

26

Cytochrome c oxidase contains 2 heme

centerscytochrome a and cytochrome a3 and two copper

proteins Each of the protein bound coppers are

associated with one of the cytochromes The copper

sites are called CuA and CuB CuA is associated with

cytochrome a and is shown to the left CuB is

associated with cytochrome a3 The copper sites

function as 1 electron carriers cycling between the

cuprous state Cu+ and the cupric state Cu2+ Just

like iron containing proteins they transfer

electrons one at a time Cytochrome c is bound on the

P-face of the membrane and transfers its electron to

CuA The oxidized cytochrome c dissociates CuA then

transfers the electron to cytochrome a The protein

bound CuA and the iron bound in cytochrome a are 15 Aring

apart In contrast the CuB and the iron bound in

cytochrome a3 are very close to each other forming a

binuclear metal center shown below

27

Electron transport chains are associated with

membranes such as the mitochondrial membrane in

eukaryotic organisms the thylakoid membrane of

photosynthetic organisms and the plasma membrane of

prokaryotic organisms While some of the details

differ electron transport chains in all organisms

operate on the same basic principles

Excited electrons are brought to the electron

transport chain by electron carriers such as NADH

and FADH2 In photosynthetic organisms the electrons

are donated by water and excited by light energy

The electron donor is oxidized as the electrons are

passed to the first protein complex in the chain As

it accepts the electrons the protein complex is

reduced Next the electrons are passed from the

28

first protein complex which becomes oxidized to

the second protein complex which becomes reduced

Each of these redox reactions is slightly exergonic

or energy releasing Some of this energy is used to

move protons across the membrane against their

concentration gradient With each successive

transfer of electrons the original excited

electrons lose some of their energy After several

successive redox reactions the electrons are

donated to a terminal electron acceptor In aerobic

respiration the terminal electron acceptor is

oxygen In photosynthesis the terminal electron

acceptor is a compound called NADP

The energy in the excited electrons has been

transformed into an electrochemical gradient

represented by the high concentration of protons on

one side of the membrane These protons can diffuse

back across the membrane by passing through a

protein complex called ATP synthase As these

protons pass through ATP synthase down their

concentration gradient they provide the energy

needed to make ATP from ADP and inorganic phosphate

within the cell

29

CHAPTER III

CLOSING

A Conclution

When a hexose is completely oxidized to CO2

and H2O using these three processes Glycolysis

yields two ATP and two NADH per hexose used Each

such NADH oxidized by the electron transport system

30

yields two ATP as dscribed above so glycolysis

contributes a total of six ATP per hexose The Krebs

cycle contributes two ATP per hexose or per two

pyruvates when succinyl CoA is cleaved to succinate

and CoASH This cycle also produces eight NADH per

hexose within the mitochondrial matrix by oxidative

phosporylation each of these NADH yields three ATP

or 24 per hexose Each FADH2 from the Krebs cycle

yields two ATP by oxydative phosporylation or four

per hexose (two pyruvates) The total contribution of

the Krebs cycle is then 30 Adding these 30 to the 6

from glycolysis leads to a total of 36 per hexose

completely respired by these process

B Suggestion

My greatest wish is that the readers find

this manuscript helpful in gaining the knowledge they

need for effective learning and practice and in

preparing for examination designed to test that

knowledge Reader comments and suggestions are

welcome and would be very helpful in improving our

manuscript Your contribution may assist many other

students in the coming years

31

REFERENCE

Anonim 2013 Anaerobic Respiration

httpwwwscoolcoukgcsebiologyrespiration

revise-itanaerobic-respiration Accessed

September 10th 2013

Anonim 2013 What is the difference between aerobic

and anaerobic respiration in plants

httpwwwbiologylifeeasyorg4010difference

-between-aerobic-anaerobic-respiration-plants

Accessed September 10th 2013

Campbell Mary K 2006 Biochemistry New York Thomson

Brooks Cole Inc

Dat J Capelli N Folzer H Bourgeade P Badot P-M

(2004) Sensing and signaling during

plantflooding Plant Physiology and

Biochemistry 42 273-282

Dat J Folzer H Parent C Badot P-M Capelli N

(2006) HypoxiastressCurrent Understanding and

Perspectives In Teixeira da Silva JA (Ed)

Floriculture Ornamental and Plant

Biotechnology Advances and Topical Issues (Vol

3) Global Science Books Isleworth United

Kingdompp 664-674

Verification of Krebs Cycle

It was done by radioactive traces like C14 as

radioactive carbon dioxide of different levels and

reactions After addition of radioactive carbon

dioxide different chemical compounds produced like

glucose fats amino acids were isolated and looked

for radioactive carbon In this way whole of

metabolic reaction were verified including individual

reactions alternative metabolic pathways

intermediates of fats carbohydrates and amino acids

etc in the body cells as well as in test tubes

(Wilson 1962 99)

The primary function of the Krebs cycle are as

follows

1 Reduction of NAD+ and FAD to the electron donors

NADH and FADH2 that are subsequently oxidized to

yield ATP

2 Direct synthesis of a limited amount of ATP (1 ATP

for each pyruvate oxidized)

3 Formation of carbon skeletons that can be used to

synthesize certain amino acids that in turn are

converted into larger molecules

11

B Fermentation

Although glycolysis can function well without

O2 the further oxidation of pyruvate and NADH by

mitochondria requires this gas Thus when O2 is

limiting NADH and pyruvate begin to accumulate

Under this condition plants carry out fermentation

(anaerobic respiration) forming either ethanol or

lactic acid (usually ethanol) As shown in the

picture below the two top reactions consists of a

decarboxylation to form acetaldehide then rapid

reduction of acetaldehide by NADH to form ethanol

These reactions are catalyzed by alcohol

dehydrogenase Some cells contain lactic acid

12

dehydrogenase which uses NADH to reduce pyruvic to

lactic acid Thus either ethanol or lactic acid or

both are fermentation products depending on the

activities of each dehydrogenase present In each

case NADH is the reductant and only under

anaerobic conditions is it abundant anough to cause

reduction Furthermore in some plants NADH is used

to cause accumulation of the other compounds when O2

is limiting especially malate and glycerol

(Salisbury 1969 233)

The fate of pyruvate produced by glycolysis

depends primarily on whether or not oxygen is

present Under normal aerobic conditions pyruvate

is transported into the mitochondrion where it is

further oxidized to CO2 and water transferring its

electrons ultimately to molecular oxygen

13

Although higher plants are obligate aerobs and

are able to tolerate anoxia for only short periods

tissues or organs are occasionally subjected to

anaerobic conditions A typical siuation is that of

roots when the soil is saturated with water When

there is no oxygen to serve as the terminal electron

acceptor mitochondrial respiration will shut down

and metabolism will shift over to fermentation

Fermentation converts pyruvate to either ethanol

through the action of the enzyme alcohol

dehydrogenase (ADH) or lactate via lactate

dehydrogenase (LDH) In most plants the principal

products of fermentation are CO2 and ethanol Some

lactate may be formed primarily in the early stages

of anoxia However lactate lowers he pH of the

cytosol which in turn activates pyruvate

decarboxylase and initiates the production of

ethanol

Either one of the fermentation reactions

consumes the NADH produced earlier in glycolysis by

the oxidation of glyceraldehide-3-P Although this

means there is no net gain of reducing potential in

fermentation This recycling of NADH is still

important to the cell The poolof NADH plus NAD+ in

the cell is relatively small and if the NADH is not

recycled there will be no supply of NAD+ to support

14

the continued oxidation of glyceraldehyde-3-P if

this were the case glycolysis and the producton of

even the small quantities of ATP necessary to

maintain the cells under anaerobic conditions would

then grind to a halt (Hopkins 2009 241)

Soil waterlogging has long been identified as a

major abiotic stress and the constraints that it

provides the roots have a significant effect on the

growth and development of plants When this event

occurs in the spring then these puddles can reduce

seed germination and seedling development Thus a

pool of water is an important factor that affects

the growth development and survival of plant

species not only in natural ecosystems but also

the agricultural and horticultural systems (Dat et

al 2006)

After flooding rapid changes in soil

properties At the time of pore water meets the

land the air pushed out gas diffusion is reduced

and toxic compounds accumulate due to anaerobic

conditions All of these changes greatly affect the

ability of plants to survive In response increased

stomatal resistance photosynthesis and root

hydraulic conductivity decreases and reduced

translocation fotoassimilat Nevertheless one of

15

the best adaptations of plants to hypoxia anoxia

is transitional biochemical and metabolic processes

common to current limited availability of O2 (Dat et

al 2004)

Selective synthesis of a set of about 20

anaerobic stress proteins (ANPS) allows the process

of producing energy without oxygen metabolism under

anaerobic conditions (Subbaiah and Sachs 2003)

Another adaptation is the observed morphological

changes consisting of the formation of hypertrophic

lenticels adventitious root initiation and or

progression of aerenchyma (Vartapetian and Jackson

1997 Jackson and Colmer 2005 Folzer et al 2006)

This review details the diverse plant stress

response to hypoxia anoxia caused by soil

waterlogging flooding and examines some of the key

features of metabolic adaptation physiological and

morphological

16

C Electron

Transport System

We noted earlier that one of the principal

functions of the respiration is to retrieve in

useful form some of the energy initially stored in

assimilates Our traditional measure of useful

energy in most processes is the number of ATP

molecules gained or consumed By this measure alone

the yield from both glycolysis and the citric acid

cycle is quite low After two complete turns of the

cycle one molecule of glucose has been completely

oxidized to six molecules of CO2 but only four

molecules of ATP have been produced (a net of two

ATP from glycolysis plus one for each turn of the

cycle) At this point most of the energy associated

with the glucose molecule has been conserved in the

17

form of electron pairs generated by the oxidation of

glycolytic and citric acid cycle intermediates

In this section we will discuss the third stage

of cellular respirationmdashthe transfer of electrons

fromNADH and FADH2 to oxygen and the accompanying

conversion of redox energy to ATP The transfer of

electrons fromNADHand FADH2 to oxygen involves a

sequence of electron carriers arranged in an

electron transport chainMembrane fractionation

studies have shown that the enzymes and electron

carriers making up the electron transport chain are

organized predominantly into four large

multimolecular complexes (complexes ImdashIV) and two

mobile carriers located in the inner mitochondrial

membrane (Figure 109) In this sense there are a

great number of similarities between the

mitochondrial inner membrane and the thylakoid

membranes of the chloroplast (compare Figure 109

with Figure 76) This is not unexpected since the

principal function of each membrane is energy

transformation and many of the same or similar

components are involved

The Components of the Electron Transport Chain

The electron transport chain of the

mitochondria is the means by which electrons are

18

removed from the reduced carrier NADH and

transferred to oxygen to yield H2O

1) NADH

NADH is generated in the matrix by the reactions

of pyruvate dehydrogenase isocitrate

dehydrogenase α-ketoglutarate dehydrogenase and

malate dehyrogenase The electron transport chain

begins with reoxidizing NADH to form NAD+ and

channeling the electrons into the formation of

reduced coenzymes Important to note that NADH

transfers 2 electrons at a time in the form of a

hydride

NAD+ + 2e- + H+ 1048581 NADH Eorsquo = minus0315 V

2) Flavoproteins

Flavoproteins have either a FAD (flavin adenosine

dinucleotide) or a FMN (flavin mononucleotide)

prosthetic group Flavoproteins can accept or

donate electrons one at time or two at a time

Thus they are often intermediaries between two

electron acceptorsdonors and one electron

acceptorsdonors For flavoproteins the typical

standard reduction potentials are around 0 V

FAD + 2e- + 2H+ 1048581 FADH2 FMN + 2e- + 2H+ 1048581 FMNH2

Eorsquo asymp 0 V

19

3) Coenzyme Q (CoQ)

aka ubiquinone (UQ) Shown below CoQ has ten

repeating isoprene units which make it

insoluble in water but soluble in the

hydrophobic lipid bylayer Coenzyme Q is a

versatile cofactor because it is a soluble

electron carrier in the hydrophobic bilipid layer

of the

inner mitochondrial membrane Like flavoproteins

CoQ can acceptdonate electrons one at a time or

two at a time

Q + 2e- + 2H+ 1048581 QH2 Eorsquo =0060 V

Q + e- + H+ 1048581 1048581QH Eorsquo =0030 V

1048581QH + e- + H+ 1048581 QH2 Eorsquo =0190 V

4) Cytochromes

Cytochromes are proteins that contain heme

prosthetic groups which function as

one electron carriers The heme iron is involved

in one electron transfers involving the Fe2+ and

Fe3+ oxidation states Cytochromes are named by

their absorption spectra which depends

on the porphyrin structure and environment The

example shown is the heme prosthetic group of

cytochrome b Cytochrome b contains the same iron

porphyrin found in hemoglobin and myoglobin

20

Other cytochromes we will encounter in the

electron transport complexes are cytochromes b

c c1 a and a3

5) Iron-Sulfur Proteins

In the electron transport chain we will encounter

many iron-sulfer proteins which participate in

one electron transfers involving the the Fe2+ and

Fe3+ oxidation states These are non-heme iron-

sulfur proteinsThe simplest iron-sulfer protein

is FeS in which iron is tetrahedrally coordinated

by four cysteines The second form is Fe2S2 which

contains two irons complexed to 2 cysteine

residues and two inorganic sulfides The third

form is Fe3S4 which contains 3 iron atoms

coordinated to three cysteine residues and 4

inorganic sulfides The last form is the most

complicated Fe4S4 which contains 4 iron atoms

coordinated to 4 cysteine residues and 4

inorganic sulfides

6) Copper Proteins

Copper bound proteins participate in one electron

transfers involving the Cu+ and Cu2+ oxidation

states

21

Overview of the Electron Transport Chain

Electrons move along the electron transport

chain going from donor to acceptor until they reach

oxygen the ultimate electron acceptor The standard

reduction potentials of the electron carriers are

between the NADHNAD+ couple (-0315 V) and the

oxygenH2O couple (0816 V) as on the next page The

components of the electron transport chain are

organized into 4 complexes Each complex contains

several different electron carriers

1 Complex I also known as the NADH-coenzyme Q

reductase or NADH dehydrogenase

2 Complex II also known as succinate-coenzyme Q

reductase or succinate dehydrogenase

3 Complex III also known as coenzyme Q reductase

4 Complex IV also known as cytochrome c reductase

The free energy necessary to generate ATP

is extracted from the oxidation of NADH and

FADH2 by the electron transport chain It

consists of four protein complexes and single

protein chain of cytochrome c through which

electrons pass from lower to higher standard

redox potentials Electrons are carried from complex

I and II to complex III by the membrane (lipid)

soluble coenzyme Q (CoQ or ubiquinone) and between

22

complexes III and IV by the peripheral membrane

protein cytochrome c

In the following I give the overall

reactions catalysed by corresponding protein

complexes in the mitochondrial electron transport

chain

1 Complex I

Complex I is also called NADH-Coenzyme Q

reductase because this large protein complex

transfers 2 electrons from NADH to coenzyme Q

Complex I was formerly known as NADH dehydrogenase

Complex I is huge 850000 kD and is composed of more

than thirty subunits It contains a FMN prosthetic

group and seven or more Fe-S clusters This complex

has between 20-26 iron atoms boundThe prosthetic

group FMN is absolutely required for activity

Therefore this complex is a flavoprotein This

complex binds NADH transfers two electrons in the

form of a hydride to FMN to produce NAD+ and FMNH2

23

The subsequent steps involve the transfer of

electrons one at a time to a series of iron-sulfer

complexes that includes both 2Fe-2S and 4Fe-4S

clusters Note the importance of FMN First it

functions as a 2 electron acceptor in the hydride

transfer from NADH Second it functions as a 1

electron donor to the series of iron sulfur clusters

FMN and FAD often play crucial links between 2

electron transfer agents and 1 electron transfer

agents The final step of this complex is the

transfer of 2 electrons one at a time to coenzyme Q

CoQ like FMN and FAD can function as a 2 electron

donoracceptor and as a 1 electron donoracceptor

CoQ is a mobile electron carrier because its

isoprenoid tail makes it highly hydrophobic and

lipophillic It diffuses freely in the bilipid layer

of the inner mitochondrial membrane

The process of transferring electrons from NADH

to CoQ by complex I results in the net transport of

protons from the matrix side of the inner

mitochondrial membrane to the inter membrane space

where the H+ ions accumulate generating a proton

motive force The intermembrane space side of the

inner membrane is referred to as the P face (P

standing for positive) The matrix side of the inner

membrane is referred to as the the N face The

24

transport of electrons from NADH to CoQ is coupled to

the transport of protons across the membrane This is

an example of active transport The stiochiometry is

4 H+ transported per 2 electrons

2 Complex II

Believe it or not you are already familiar

with Complex II It is none other than succinate

dehydrogenase The only enzyme of the citric acid

cycle that is an integral membrane protein This

complex is composed of four subunits 2 of which are

iron-sulfur proteins and the other two subunits

together bind FAD through a covalent link to a

histidine residue These two subunits are called

flavoprotein 2 or FP2 Complex II contains 3 Fe-S

centers 1 4Fe-4S cluster 1 3Fe-4S cluster and 1

2Fe- 2S cluster In the first step of this complex

succinate is bound and a hydride is transferred to

FAD to generate FADH2 and fumarate FADH2 then

transfers its electrons one at a time to the Fe-S

centers Thus once again FAD functions as 2 electron

acceptor and a 1 electron donor The final step of

this complex is the transfer of 2 electrons one at a

time to coenzyme Q to produce CoQH2 For complex II

the standard free energy change of the overall

reaction is too small to drive the transport of

protons across the inner mitochondrial membrane This

25

accounts for the 15 ATPrsquos generated per FADH2

compared with the 25 ATPrsquos generated per NADH

3 Complex III

This complex is also known as coenzyme Q-

cytochrome c reductase because it passes the

electrons form CoQH2 to cyt c through a very unique

electron transport pathway called the Q-cycle Shown

to the left are the porphyrins found in cytochromes

Cytochrome b contains the same iron protoporphyrin as

hemoglobin and myoglobin The c cytochromes contain

heme c through covalent attachment by cysteine

residues Cytochrome a is found in two forms in

complex IV In complex III we find two b-type

cytochromes and one c-type cytochrome Complex III is

complex and we have a crystal structure

4 Complex IV

Complex IV is also known as cytochrome c oxidase

because it accepts the electrons from cytochrome c

and directs them towards the four electron reduction

of O2 to form 2 molecules of H2O

4 cyt c (Fe2+) + 4 H+ + O2 4 cyt c (Fe3+) +

2H2O

26

Cytochrome c oxidase contains 2 heme

centerscytochrome a and cytochrome a3 and two copper

proteins Each of the protein bound coppers are

associated with one of the cytochromes The copper

sites are called CuA and CuB CuA is associated with

cytochrome a and is shown to the left CuB is

associated with cytochrome a3 The copper sites

function as 1 electron carriers cycling between the

cuprous state Cu+ and the cupric state Cu2+ Just

like iron containing proteins they transfer

electrons one at a time Cytochrome c is bound on the

P-face of the membrane and transfers its electron to

CuA The oxidized cytochrome c dissociates CuA then

transfers the electron to cytochrome a The protein

bound CuA and the iron bound in cytochrome a are 15 Aring

apart In contrast the CuB and the iron bound in

cytochrome a3 are very close to each other forming a

binuclear metal center shown below

27

Electron transport chains are associated with

membranes such as the mitochondrial membrane in

eukaryotic organisms the thylakoid membrane of

photosynthetic organisms and the plasma membrane of

prokaryotic organisms While some of the details

differ electron transport chains in all organisms

operate on the same basic principles

Excited electrons are brought to the electron

transport chain by electron carriers such as NADH

and FADH2 In photosynthetic organisms the electrons

are donated by water and excited by light energy

The electron donor is oxidized as the electrons are

passed to the first protein complex in the chain As

it accepts the electrons the protein complex is

reduced Next the electrons are passed from the

28

first protein complex which becomes oxidized to

the second protein complex which becomes reduced

Each of these redox reactions is slightly exergonic

or energy releasing Some of this energy is used to

move protons across the membrane against their

concentration gradient With each successive

transfer of electrons the original excited

electrons lose some of their energy After several

successive redox reactions the electrons are

donated to a terminal electron acceptor In aerobic

respiration the terminal electron acceptor is

oxygen In photosynthesis the terminal electron

acceptor is a compound called NADP

The energy in the excited electrons has been

transformed into an electrochemical gradient

represented by the high concentration of protons on

one side of the membrane These protons can diffuse

back across the membrane by passing through a

protein complex called ATP synthase As these

protons pass through ATP synthase down their

concentration gradient they provide the energy

needed to make ATP from ADP and inorganic phosphate

within the cell

29

CHAPTER III

CLOSING

A Conclution

When a hexose is completely oxidized to CO2

and H2O using these three processes Glycolysis

yields two ATP and two NADH per hexose used Each

such NADH oxidized by the electron transport system

30

yields two ATP as dscribed above so glycolysis

contributes a total of six ATP per hexose The Krebs

cycle contributes two ATP per hexose or per two

pyruvates when succinyl CoA is cleaved to succinate

and CoASH This cycle also produces eight NADH per

hexose within the mitochondrial matrix by oxidative

phosporylation each of these NADH yields three ATP

or 24 per hexose Each FADH2 from the Krebs cycle

yields two ATP by oxydative phosporylation or four

per hexose (two pyruvates) The total contribution of

the Krebs cycle is then 30 Adding these 30 to the 6

from glycolysis leads to a total of 36 per hexose

completely respired by these process

B Suggestion

My greatest wish is that the readers find

this manuscript helpful in gaining the knowledge they

need for effective learning and practice and in

preparing for examination designed to test that

knowledge Reader comments and suggestions are

welcome and would be very helpful in improving our

manuscript Your contribution may assist many other

students in the coming years

31

REFERENCE

Anonim 2013 Anaerobic Respiration

httpwwwscoolcoukgcsebiologyrespiration

revise-itanaerobic-respiration Accessed

September 10th 2013

Anonim 2013 What is the difference between aerobic

and anaerobic respiration in plants

httpwwwbiologylifeeasyorg4010difference

-between-aerobic-anaerobic-respiration-plants

Accessed September 10th 2013

Campbell Mary K 2006 Biochemistry New York Thomson

Brooks Cole Inc

Dat J Capelli N Folzer H Bourgeade P Badot P-M

(2004) Sensing and signaling during

plantflooding Plant Physiology and

Biochemistry 42 273-282

Dat J Folzer H Parent C Badot P-M Capelli N

(2006) HypoxiastressCurrent Understanding and

Perspectives In Teixeira da Silva JA (Ed)

Floriculture Ornamental and Plant

Biotechnology Advances and Topical Issues (Vol

3) Global Science Books Isleworth United

Kingdompp 664-674

B Fermentation

Although glycolysis can function well without

O2 the further oxidation of pyruvate and NADH by

mitochondria requires this gas Thus when O2 is

limiting NADH and pyruvate begin to accumulate

Under this condition plants carry out fermentation

(anaerobic respiration) forming either ethanol or

lactic acid (usually ethanol) As shown in the

picture below the two top reactions consists of a

decarboxylation to form acetaldehide then rapid

reduction of acetaldehide by NADH to form ethanol

These reactions are catalyzed by alcohol

dehydrogenase Some cells contain lactic acid

12

dehydrogenase which uses NADH to reduce pyruvic to

lactic acid Thus either ethanol or lactic acid or

both are fermentation products depending on the

activities of each dehydrogenase present In each

case NADH is the reductant and only under

anaerobic conditions is it abundant anough to cause

reduction Furthermore in some plants NADH is used

to cause accumulation of the other compounds when O2

is limiting especially malate and glycerol

(Salisbury 1969 233)

The fate of pyruvate produced by glycolysis

depends primarily on whether or not oxygen is

present Under normal aerobic conditions pyruvate

is transported into the mitochondrion where it is

further oxidized to CO2 and water transferring its

electrons ultimately to molecular oxygen

13

Although higher plants are obligate aerobs and

are able to tolerate anoxia for only short periods

tissues or organs are occasionally subjected to

anaerobic conditions A typical siuation is that of

roots when the soil is saturated with water When

there is no oxygen to serve as the terminal electron

acceptor mitochondrial respiration will shut down

and metabolism will shift over to fermentation

Fermentation converts pyruvate to either ethanol

through the action of the enzyme alcohol

dehydrogenase (ADH) or lactate via lactate

dehydrogenase (LDH) In most plants the principal

products of fermentation are CO2 and ethanol Some

lactate may be formed primarily in the early stages

of anoxia However lactate lowers he pH of the

cytosol which in turn activates pyruvate

decarboxylase and initiates the production of

ethanol

Either one of the fermentation reactions

consumes the NADH produced earlier in glycolysis by

the oxidation of glyceraldehide-3-P Although this

means there is no net gain of reducing potential in

fermentation This recycling of NADH is still

important to the cell The poolof NADH plus NAD+ in

the cell is relatively small and if the NADH is not

recycled there will be no supply of NAD+ to support

14

the continued oxidation of glyceraldehyde-3-P if

this were the case glycolysis and the producton of

even the small quantities of ATP necessary to

maintain the cells under anaerobic conditions would

then grind to a halt (Hopkins 2009 241)

Soil waterlogging has long been identified as a

major abiotic stress and the constraints that it

provides the roots have a significant effect on the

growth and development of plants When this event

occurs in the spring then these puddles can reduce

seed germination and seedling development Thus a

pool of water is an important factor that affects

the growth development and survival of plant

species not only in natural ecosystems but also

the agricultural and horticultural systems (Dat et

al 2006)

After flooding rapid changes in soil

properties At the time of pore water meets the

land the air pushed out gas diffusion is reduced

and toxic compounds accumulate due to anaerobic

conditions All of these changes greatly affect the

ability of plants to survive In response increased

stomatal resistance photosynthesis and root

hydraulic conductivity decreases and reduced

translocation fotoassimilat Nevertheless one of

15

the best adaptations of plants to hypoxia anoxia

is transitional biochemical and metabolic processes

common to current limited availability of O2 (Dat et

al 2004)

Selective synthesis of a set of about 20

anaerobic stress proteins (ANPS) allows the process

of producing energy without oxygen metabolism under

anaerobic conditions (Subbaiah and Sachs 2003)

Another adaptation is the observed morphological

changes consisting of the formation of hypertrophic

lenticels adventitious root initiation and or

progression of aerenchyma (Vartapetian and Jackson

1997 Jackson and Colmer 2005 Folzer et al 2006)

This review details the diverse plant stress

response to hypoxia anoxia caused by soil

waterlogging flooding and examines some of the key

features of metabolic adaptation physiological and

morphological

16

C Electron

Transport System

We noted earlier that one of the principal

functions of the respiration is to retrieve in

useful form some of the energy initially stored in

assimilates Our traditional measure of useful

energy in most processes is the number of ATP

molecules gained or consumed By this measure alone

the yield from both glycolysis and the citric acid

cycle is quite low After two complete turns of the

cycle one molecule of glucose has been completely

oxidized to six molecules of CO2 but only four

molecules of ATP have been produced (a net of two

ATP from glycolysis plus one for each turn of the

cycle) At this point most of the energy associated

with the glucose molecule has been conserved in the

17

form of electron pairs generated by the oxidation of

glycolytic and citric acid cycle intermediates

In this section we will discuss the third stage

of cellular respirationmdashthe transfer of electrons

fromNADH and FADH2 to oxygen and the accompanying

conversion of redox energy to ATP The transfer of

electrons fromNADHand FADH2 to oxygen involves a

sequence of electron carriers arranged in an

electron transport chainMembrane fractionation

studies have shown that the enzymes and electron

carriers making up the electron transport chain are

organized predominantly into four large

multimolecular complexes (complexes ImdashIV) and two

mobile carriers located in the inner mitochondrial

membrane (Figure 109) In this sense there are a

great number of similarities between the

mitochondrial inner membrane and the thylakoid

membranes of the chloroplast (compare Figure 109

with Figure 76) This is not unexpected since the

principal function of each membrane is energy

transformation and many of the same or similar

components are involved

The Components of the Electron Transport Chain

The electron transport chain of the

mitochondria is the means by which electrons are

18

removed from the reduced carrier NADH and

transferred to oxygen to yield H2O

1) NADH

NADH is generated in the matrix by the reactions

of pyruvate dehydrogenase isocitrate

dehydrogenase α-ketoglutarate dehydrogenase and

malate dehyrogenase The electron transport chain

begins with reoxidizing NADH to form NAD+ and

channeling the electrons into the formation of

reduced coenzymes Important to note that NADH

transfers 2 electrons at a time in the form of a

hydride

NAD+ + 2e- + H+ 1048581 NADH Eorsquo = minus0315 V

2) Flavoproteins

Flavoproteins have either a FAD (flavin adenosine

dinucleotide) or a FMN (flavin mononucleotide)

prosthetic group Flavoproteins can accept or

donate electrons one at time or two at a time

Thus they are often intermediaries between two

electron acceptorsdonors and one electron

acceptorsdonors For flavoproteins the typical

standard reduction potentials are around 0 V

FAD + 2e- + 2H+ 1048581 FADH2 FMN + 2e- + 2H+ 1048581 FMNH2

Eorsquo asymp 0 V

19

3) Coenzyme Q (CoQ)

aka ubiquinone (UQ) Shown below CoQ has ten

repeating isoprene units which make it

insoluble in water but soluble in the

hydrophobic lipid bylayer Coenzyme Q is a

versatile cofactor because it is a soluble

electron carrier in the hydrophobic bilipid layer

of the

inner mitochondrial membrane Like flavoproteins

CoQ can acceptdonate electrons one at a time or

two at a time

Q + 2e- + 2H+ 1048581 QH2 Eorsquo =0060 V

Q + e- + H+ 1048581 1048581QH Eorsquo =0030 V

1048581QH + e- + H+ 1048581 QH2 Eorsquo =0190 V

4) Cytochromes

Cytochromes are proteins that contain heme

prosthetic groups which function as

one electron carriers The heme iron is involved

in one electron transfers involving the Fe2+ and

Fe3+ oxidation states Cytochromes are named by

their absorption spectra which depends

on the porphyrin structure and environment The

example shown is the heme prosthetic group of

cytochrome b Cytochrome b contains the same iron

porphyrin found in hemoglobin and myoglobin

20

Other cytochromes we will encounter in the

electron transport complexes are cytochromes b

c c1 a and a3

5) Iron-Sulfur Proteins

In the electron transport chain we will encounter

many iron-sulfer proteins which participate in

one electron transfers involving the the Fe2+ and

Fe3+ oxidation states These are non-heme iron-

sulfur proteinsThe simplest iron-sulfer protein

is FeS in which iron is tetrahedrally coordinated

by four cysteines The second form is Fe2S2 which

contains two irons complexed to 2 cysteine

residues and two inorganic sulfides The third

form is Fe3S4 which contains 3 iron atoms

coordinated to three cysteine residues and 4

inorganic sulfides The last form is the most

complicated Fe4S4 which contains 4 iron atoms

coordinated to 4 cysteine residues and 4

inorganic sulfides

6) Copper Proteins

Copper bound proteins participate in one electron

transfers involving the Cu+ and Cu2+ oxidation

states

21

Overview of the Electron Transport Chain

Electrons move along the electron transport

chain going from donor to acceptor until they reach

oxygen the ultimate electron acceptor The standard

reduction potentials of the electron carriers are

between the NADHNAD+ couple (-0315 V) and the

oxygenH2O couple (0816 V) as on the next page The

components of the electron transport chain are

organized into 4 complexes Each complex contains

several different electron carriers

1 Complex I also known as the NADH-coenzyme Q

reductase or NADH dehydrogenase

2 Complex II also known as succinate-coenzyme Q

reductase or succinate dehydrogenase

3 Complex III also known as coenzyme Q reductase

4 Complex IV also known as cytochrome c reductase

The free energy necessary to generate ATP

is extracted from the oxidation of NADH and

FADH2 by the electron transport chain It

consists of four protein complexes and single

protein chain of cytochrome c through which

electrons pass from lower to higher standard

redox potentials Electrons are carried from complex

I and II to complex III by the membrane (lipid)

soluble coenzyme Q (CoQ or ubiquinone) and between

22

complexes III and IV by the peripheral membrane

protein cytochrome c

In the following I give the overall

reactions catalysed by corresponding protein

complexes in the mitochondrial electron transport

chain

1 Complex I

Complex I is also called NADH-Coenzyme Q

reductase because this large protein complex

transfers 2 electrons from NADH to coenzyme Q

Complex I was formerly known as NADH dehydrogenase

Complex I is huge 850000 kD and is composed of more

than thirty subunits It contains a FMN prosthetic

group and seven or more Fe-S clusters This complex

has between 20-26 iron atoms boundThe prosthetic

group FMN is absolutely required for activity

Therefore this complex is a flavoprotein This

complex binds NADH transfers two electrons in the

form of a hydride to FMN to produce NAD+ and FMNH2

23

The subsequent steps involve the transfer of

electrons one at a time to a series of iron-sulfer

complexes that includes both 2Fe-2S and 4Fe-4S

clusters Note the importance of FMN First it

functions as a 2 electron acceptor in the hydride

transfer from NADH Second it functions as a 1

electron donor to the series of iron sulfur clusters

FMN and FAD often play crucial links between 2

electron transfer agents and 1 electron transfer

agents The final step of this complex is the

transfer of 2 electrons one at a time to coenzyme Q

CoQ like FMN and FAD can function as a 2 electron

donoracceptor and as a 1 electron donoracceptor

CoQ is a mobile electron carrier because its

isoprenoid tail makes it highly hydrophobic and

lipophillic It diffuses freely in the bilipid layer

of the inner mitochondrial membrane

The process of transferring electrons from NADH

to CoQ by complex I results in the net transport of

protons from the matrix side of the inner

mitochondrial membrane to the inter membrane space

where the H+ ions accumulate generating a proton

motive force The intermembrane space side of the

inner membrane is referred to as the P face (P

standing for positive) The matrix side of the inner

membrane is referred to as the the N face The

24

transport of electrons from NADH to CoQ is coupled to

the transport of protons across the membrane This is

an example of active transport The stiochiometry is

4 H+ transported per 2 electrons

2 Complex II

Believe it or not you are already familiar

with Complex II It is none other than succinate

dehydrogenase The only enzyme of the citric acid

cycle that is an integral membrane protein This

complex is composed of four subunits 2 of which are

iron-sulfur proteins and the other two subunits

together bind FAD through a covalent link to a

histidine residue These two subunits are called

flavoprotein 2 or FP2 Complex II contains 3 Fe-S

centers 1 4Fe-4S cluster 1 3Fe-4S cluster and 1

2Fe- 2S cluster In the first step of this complex

succinate is bound and a hydride is transferred to

FAD to generate FADH2 and fumarate FADH2 then

transfers its electrons one at a time to the Fe-S

centers Thus once again FAD functions as 2 electron

acceptor and a 1 electron donor The final step of

this complex is the transfer of 2 electrons one at a

time to coenzyme Q to produce CoQH2 For complex II

the standard free energy change of the overall

reaction is too small to drive the transport of

protons across the inner mitochondrial membrane This

25

accounts for the 15 ATPrsquos generated per FADH2

compared with the 25 ATPrsquos generated per NADH

3 Complex III

This complex is also known as coenzyme Q-

cytochrome c reductase because it passes the

electrons form CoQH2 to cyt c through a very unique

electron transport pathway called the Q-cycle Shown

to the left are the porphyrins found in cytochromes

Cytochrome b contains the same iron protoporphyrin as

hemoglobin and myoglobin The c cytochromes contain

heme c through covalent attachment by cysteine

residues Cytochrome a is found in two forms in

complex IV In complex III we find two b-type

cytochromes and one c-type cytochrome Complex III is

complex and we have a crystal structure

4 Complex IV

Complex IV is also known as cytochrome c oxidase

because it accepts the electrons from cytochrome c

and directs them towards the four electron reduction

of O2 to form 2 molecules of H2O

4 cyt c (Fe2+) + 4 H+ + O2 4 cyt c (Fe3+) +

2H2O

26

Cytochrome c oxidase contains 2 heme

centerscytochrome a and cytochrome a3 and two copper

proteins Each of the protein bound coppers are

associated with one of the cytochromes The copper

sites are called CuA and CuB CuA is associated with

cytochrome a and is shown to the left CuB is

associated with cytochrome a3 The copper sites

function as 1 electron carriers cycling between the

cuprous state Cu+ and the cupric state Cu2+ Just

like iron containing proteins they transfer

electrons one at a time Cytochrome c is bound on the

P-face of the membrane and transfers its electron to

CuA The oxidized cytochrome c dissociates CuA then

transfers the electron to cytochrome a The protein

bound CuA and the iron bound in cytochrome a are 15 Aring

apart In contrast the CuB and the iron bound in

cytochrome a3 are very close to each other forming a

binuclear metal center shown below

27

Electron transport chains are associated with

membranes such as the mitochondrial membrane in

eukaryotic organisms the thylakoid membrane of

photosynthetic organisms and the plasma membrane of

prokaryotic organisms While some of the details

differ electron transport chains in all organisms

operate on the same basic principles

Excited electrons are brought to the electron

transport chain by electron carriers such as NADH

and FADH2 In photosynthetic organisms the electrons

are donated by water and excited by light energy

The electron donor is oxidized as the electrons are

passed to the first protein complex in the chain As

it accepts the electrons the protein complex is

reduced Next the electrons are passed from the

28

first protein complex which becomes oxidized to

the second protein complex which becomes reduced

Each of these redox reactions is slightly exergonic

or energy releasing Some of this energy is used to

move protons across the membrane against their

concentration gradient With each successive

transfer of electrons the original excited

electrons lose some of their energy After several

successive redox reactions the electrons are

donated to a terminal electron acceptor In aerobic

respiration the terminal electron acceptor is

oxygen In photosynthesis the terminal electron

acceptor is a compound called NADP

The energy in the excited electrons has been

transformed into an electrochemical gradient

represented by the high concentration of protons on

one side of the membrane These protons can diffuse

back across the membrane by passing through a

protein complex called ATP synthase As these

protons pass through ATP synthase down their

concentration gradient they provide the energy

needed to make ATP from ADP and inorganic phosphate

within the cell

29

CHAPTER III

CLOSING

A Conclution

When a hexose is completely oxidized to CO2

and H2O using these three processes Glycolysis

yields two ATP and two NADH per hexose used Each

such NADH oxidized by the electron transport system

30

yields two ATP as dscribed above so glycolysis

contributes a total of six ATP per hexose The Krebs

cycle contributes two ATP per hexose or per two

pyruvates when succinyl CoA is cleaved to succinate

and CoASH This cycle also produces eight NADH per

hexose within the mitochondrial matrix by oxidative

phosporylation each of these NADH yields three ATP

or 24 per hexose Each FADH2 from the Krebs cycle

yields two ATP by oxydative phosporylation or four

per hexose (two pyruvates) The total contribution of

the Krebs cycle is then 30 Adding these 30 to the 6

from glycolysis leads to a total of 36 per hexose

completely respired by these process

B Suggestion

My greatest wish is that the readers find

this manuscript helpful in gaining the knowledge they

need for effective learning and practice and in

preparing for examination designed to test that

knowledge Reader comments and suggestions are

welcome and would be very helpful in improving our

manuscript Your contribution may assist many other

students in the coming years

31

REFERENCE

Anonim 2013 Anaerobic Respiration

httpwwwscoolcoukgcsebiologyrespiration

revise-itanaerobic-respiration Accessed

September 10th 2013

Anonim 2013 What is the difference between aerobic

and anaerobic respiration in plants

httpwwwbiologylifeeasyorg4010difference

-between-aerobic-anaerobic-respiration-plants

Accessed September 10th 2013

Campbell Mary K 2006 Biochemistry New York Thomson

Brooks Cole Inc

Dat J Capelli N Folzer H Bourgeade P Badot P-M

(2004) Sensing and signaling during

plantflooding Plant Physiology and

Biochemistry 42 273-282

Dat J Folzer H Parent C Badot P-M Capelli N

(2006) HypoxiastressCurrent Understanding and

Perspectives In Teixeira da Silva JA (Ed)

Floriculture Ornamental and Plant

Biotechnology Advances and Topical Issues (Vol

3) Global Science Books Isleworth United

Kingdompp 664-674

dehydrogenase which uses NADH to reduce pyruvic to

lactic acid Thus either ethanol or lactic acid or

both are fermentation products depending on the

activities of each dehydrogenase present In each

case NADH is the reductant and only under

anaerobic conditions is it abundant anough to cause

reduction Furthermore in some plants NADH is used

to cause accumulation of the other compounds when O2

is limiting especially malate and glycerol

(Salisbury 1969 233)

The fate of pyruvate produced by glycolysis

depends primarily on whether or not oxygen is

present Under normal aerobic conditions pyruvate

is transported into the mitochondrion where it is

further oxidized to CO2 and water transferring its

electrons ultimately to molecular oxygen

13

Although higher plants are obligate aerobs and

are able to tolerate anoxia for only short periods

tissues or organs are occasionally subjected to

anaerobic conditions A typical siuation is that of

roots when the soil is saturated with water When

there is no oxygen to serve as the terminal electron

acceptor mitochondrial respiration will shut down

and metabolism will shift over to fermentation

Fermentation converts pyruvate to either ethanol

through the action of the enzyme alcohol

dehydrogenase (ADH) or lactate via lactate

dehydrogenase (LDH) In most plants the principal

products of fermentation are CO2 and ethanol Some

lactate may be formed primarily in the early stages

of anoxia However lactate lowers he pH of the

cytosol which in turn activates pyruvate

decarboxylase and initiates the production of

ethanol

Either one of the fermentation reactions

consumes the NADH produced earlier in glycolysis by

the oxidation of glyceraldehide-3-P Although this

means there is no net gain of reducing potential in

fermentation This recycling of NADH is still

important to the cell The poolof NADH plus NAD+ in

the cell is relatively small and if the NADH is not

recycled there will be no supply of NAD+ to support

14

the continued oxidation of glyceraldehyde-3-P if

this were the case glycolysis and the producton of

even the small quantities of ATP necessary to

maintain the cells under anaerobic conditions would

then grind to a halt (Hopkins 2009 241)

Soil waterlogging has long been identified as a

major abiotic stress and the constraints that it

provides the roots have a significant effect on the

growth and development of plants When this event

occurs in the spring then these puddles can reduce

seed germination and seedling development Thus a

pool of water is an important factor that affects

the growth development and survival of plant

species not only in natural ecosystems but also

the agricultural and horticultural systems (Dat et

al 2006)

After flooding rapid changes in soil

properties At the time of pore water meets the

land the air pushed out gas diffusion is reduced

and toxic compounds accumulate due to anaerobic

conditions All of these changes greatly affect the

ability of plants to survive In response increased

stomatal resistance photosynthesis and root

hydraulic conductivity decreases and reduced

translocation fotoassimilat Nevertheless one of

15

the best adaptations of plants to hypoxia anoxia

is transitional biochemical and metabolic processes

common to current limited availability of O2 (Dat et

al 2004)

Selective synthesis of a set of about 20

anaerobic stress proteins (ANPS) allows the process

of producing energy without oxygen metabolism under

anaerobic conditions (Subbaiah and Sachs 2003)

Another adaptation is the observed morphological

changes consisting of the formation of hypertrophic

lenticels adventitious root initiation and or

progression of aerenchyma (Vartapetian and Jackson

1997 Jackson and Colmer 2005 Folzer et al 2006)

This review details the diverse plant stress

response to hypoxia anoxia caused by soil

waterlogging flooding and examines some of the key

features of metabolic adaptation physiological and

morphological

16

C Electron

Transport System

We noted earlier that one of the principal

functions of the respiration is to retrieve in

useful form some of the energy initially stored in

assimilates Our traditional measure of useful

energy in most processes is the number of ATP

molecules gained or consumed By this measure alone

the yield from both glycolysis and the citric acid

cycle is quite low After two complete turns of the

cycle one molecule of glucose has been completely

oxidized to six molecules of CO2 but only four

molecules of ATP have been produced (a net of two

ATP from glycolysis plus one for each turn of the

cycle) At this point most of the energy associated

with the glucose molecule has been conserved in the

17

form of electron pairs generated by the oxidation of

glycolytic and citric acid cycle intermediates

In this section we will discuss the third stage

of cellular respirationmdashthe transfer of electrons

fromNADH and FADH2 to oxygen and the accompanying

conversion of redox energy to ATP The transfer of

electrons fromNADHand FADH2 to oxygen involves a

sequence of electron carriers arranged in an

electron transport chainMembrane fractionation

studies have shown that the enzymes and electron

carriers making up the electron transport chain are

organized predominantly into four large

multimolecular complexes (complexes ImdashIV) and two

mobile carriers located in the inner mitochondrial

membrane (Figure 109) In this sense there are a

great number of similarities between the

mitochondrial inner membrane and the thylakoid

membranes of the chloroplast (compare Figure 109

with Figure 76) This is not unexpected since the

principal function of each membrane is energy

transformation and many of the same or similar

components are involved

The Components of the Electron Transport Chain

The electron transport chain of the

mitochondria is the means by which electrons are

18

removed from the reduced carrier NADH and

transferred to oxygen to yield H2O

1) NADH

NADH is generated in the matrix by the reactions

of pyruvate dehydrogenase isocitrate

dehydrogenase α-ketoglutarate dehydrogenase and

malate dehyrogenase The electron transport chain

begins with reoxidizing NADH to form NAD+ and

channeling the electrons into the formation of

reduced coenzymes Important to note that NADH

transfers 2 electrons at a time in the form of a

hydride

NAD+ + 2e- + H+ 1048581 NADH Eorsquo = minus0315 V

2) Flavoproteins

Flavoproteins have either a FAD (flavin adenosine

dinucleotide) or a FMN (flavin mononucleotide)

prosthetic group Flavoproteins can accept or

donate electrons one at time or two at a time

Thus they are often intermediaries between two

electron acceptorsdonors and one electron

acceptorsdonors For flavoproteins the typical

standard reduction potentials are around 0 V

FAD + 2e- + 2H+ 1048581 FADH2 FMN + 2e- + 2H+ 1048581 FMNH2

Eorsquo asymp 0 V

19

3) Coenzyme Q (CoQ)

aka ubiquinone (UQ) Shown below CoQ has ten

repeating isoprene units which make it

insoluble in water but soluble in the

hydrophobic lipid bylayer Coenzyme Q is a

versatile cofactor because it is a soluble

electron carrier in the hydrophobic bilipid layer

of the

inner mitochondrial membrane Like flavoproteins

CoQ can acceptdonate electrons one at a time or

two at a time

Q + 2e- + 2H+ 1048581 QH2 Eorsquo =0060 V

Q + e- + H+ 1048581 1048581QH Eorsquo =0030 V

1048581QH + e- + H+ 1048581 QH2 Eorsquo =0190 V

4) Cytochromes

Cytochromes are proteins that contain heme

prosthetic groups which function as

one electron carriers The heme iron is involved

in one electron transfers involving the Fe2+ and

Fe3+ oxidation states Cytochromes are named by

their absorption spectra which depends

on the porphyrin structure and environment The

example shown is the heme prosthetic group of

cytochrome b Cytochrome b contains the same iron

porphyrin found in hemoglobin and myoglobin

20

Other cytochromes we will encounter in the

electron transport complexes are cytochromes b

c c1 a and a3

5) Iron-Sulfur Proteins

In the electron transport chain we will encounter

many iron-sulfer proteins which participate in

one electron transfers involving the the Fe2+ and

Fe3+ oxidation states These are non-heme iron-

sulfur proteinsThe simplest iron-sulfer protein

is FeS in which iron is tetrahedrally coordinated

by four cysteines The second form is Fe2S2 which

contains two irons complexed to 2 cysteine

residues and two inorganic sulfides The third

form is Fe3S4 which contains 3 iron atoms

coordinated to three cysteine residues and 4

inorganic sulfides The last form is the most

complicated Fe4S4 which contains 4 iron atoms

coordinated to 4 cysteine residues and 4

inorganic sulfides

6) Copper Proteins

Copper bound proteins participate in one electron

transfers involving the Cu+ and Cu2+ oxidation

states

21

Overview of the Electron Transport Chain

Electrons move along the electron transport

chain going from donor to acceptor until they reach

oxygen the ultimate electron acceptor The standard

reduction potentials of the electron carriers are

between the NADHNAD+ couple (-0315 V) and the

oxygenH2O couple (0816 V) as on the next page The

components of the electron transport chain are

organized into 4 complexes Each complex contains

several different electron carriers

1 Complex I also known as the NADH-coenzyme Q

reductase or NADH dehydrogenase

2 Complex II also known as succinate-coenzyme Q

reductase or succinate dehydrogenase

3 Complex III also known as coenzyme Q reductase

4 Complex IV also known as cytochrome c reductase

The free energy necessary to generate ATP

is extracted from the oxidation of NADH and

FADH2 by the electron transport chain It

consists of four protein complexes and single

protein chain of cytochrome c through which

electrons pass from lower to higher standard

redox potentials Electrons are carried from complex

I and II to complex III by the membrane (lipid)

soluble coenzyme Q (CoQ or ubiquinone) and between

22

complexes III and IV by the peripheral membrane

protein cytochrome c

In the following I give the overall

reactions catalysed by corresponding protein

complexes in the mitochondrial electron transport

chain

1 Complex I

Complex I is also called NADH-Coenzyme Q

reductase because this large protein complex

transfers 2 electrons from NADH to coenzyme Q

Complex I was formerly known as NADH dehydrogenase

Complex I is huge 850000 kD and is composed of more

than thirty subunits It contains a FMN prosthetic

group and seven or more Fe-S clusters This complex

has between 20-26 iron atoms boundThe prosthetic

group FMN is absolutely required for activity

Therefore this complex is a flavoprotein This

complex binds NADH transfers two electrons in the

form of a hydride to FMN to produce NAD+ and FMNH2

23

The subsequent steps involve the transfer of

electrons one at a time to a series of iron-sulfer

complexes that includes both 2Fe-2S and 4Fe-4S

clusters Note the importance of FMN First it

functions as a 2 electron acceptor in the hydride

transfer from NADH Second it functions as a 1

electron donor to the series of iron sulfur clusters

FMN and FAD often play crucial links between 2

electron transfer agents and 1 electron transfer

agents The final step of this complex is the

transfer of 2 electrons one at a time to coenzyme Q

CoQ like FMN and FAD can function as a 2 electron

donoracceptor and as a 1 electron donoracceptor

CoQ is a mobile electron carrier because its

isoprenoid tail makes it highly hydrophobic and

lipophillic It diffuses freely in the bilipid layer

of the inner mitochondrial membrane

The process of transferring electrons from NADH

to CoQ by complex I results in the net transport of

protons from the matrix side of the inner

mitochondrial membrane to the inter membrane space

where the H+ ions accumulate generating a proton

motive force The intermembrane space side of the

inner membrane is referred to as the P face (P

standing for positive) The matrix side of the inner

membrane is referred to as the the N face The

24

transport of electrons from NADH to CoQ is coupled to

the transport of protons across the membrane This is

an example of active transport The stiochiometry is

4 H+ transported per 2 electrons

2 Complex II

Believe it or not you are already familiar

with Complex II It is none other than succinate

dehydrogenase The only enzyme of the citric acid

cycle that is an integral membrane protein This

complex is composed of four subunits 2 of which are

iron-sulfur proteins and the other two subunits

together bind FAD through a covalent link to a

histidine residue These two subunits are called

flavoprotein 2 or FP2 Complex II contains 3 Fe-S

centers 1 4Fe-4S cluster 1 3Fe-4S cluster and 1

2Fe- 2S cluster In the first step of this complex

succinate is bound and a hydride is transferred to

FAD to generate FADH2 and fumarate FADH2 then

transfers its electrons one at a time to the Fe-S

centers Thus once again FAD functions as 2 electron

acceptor and a 1 electron donor The final step of

this complex is the transfer of 2 electrons one at a

time to coenzyme Q to produce CoQH2 For complex II

the standard free energy change of the overall

reaction is too small to drive the transport of

protons across the inner mitochondrial membrane This

25

accounts for the 15 ATPrsquos generated per FADH2

compared with the 25 ATPrsquos generated per NADH

3 Complex III

This complex is also known as coenzyme Q-

cytochrome c reductase because it passes the

electrons form CoQH2 to cyt c through a very unique

electron transport pathway called the Q-cycle Shown

to the left are the porphyrins found in cytochromes

Cytochrome b contains the same iron protoporphyrin as

hemoglobin and myoglobin The c cytochromes contain

heme c through covalent attachment by cysteine

residues Cytochrome a is found in two forms in

complex IV In complex III we find two b-type

cytochromes and one c-type cytochrome Complex III is

complex and we have a crystal structure

4 Complex IV

Complex IV is also known as cytochrome c oxidase

because it accepts the electrons from cytochrome c

and directs them towards the four electron reduction

of O2 to form 2 molecules of H2O

4 cyt c (Fe2+) + 4 H+ + O2 4 cyt c (Fe3+) +

2H2O

26

Cytochrome c oxidase contains 2 heme

centerscytochrome a and cytochrome a3 and two copper

proteins Each of the protein bound coppers are

associated with one of the cytochromes The copper

sites are called CuA and CuB CuA is associated with

cytochrome a and is shown to the left CuB is

associated with cytochrome a3 The copper sites

function as 1 electron carriers cycling between the

cuprous state Cu+ and the cupric state Cu2+ Just

like iron containing proteins they transfer

electrons one at a time Cytochrome c is bound on the

P-face of the membrane and transfers its electron to

CuA The oxidized cytochrome c dissociates CuA then

transfers the electron to cytochrome a The protein

bound CuA and the iron bound in cytochrome a are 15 Aring

apart In contrast the CuB and the iron bound in

cytochrome a3 are very close to each other forming a

binuclear metal center shown below

27

Electron transport chains are associated with

membranes such as the mitochondrial membrane in

eukaryotic organisms the thylakoid membrane of

photosynthetic organisms and the plasma membrane of

prokaryotic organisms While some of the details

differ electron transport chains in all organisms

operate on the same basic principles

Excited electrons are brought to the electron

transport chain by electron carriers such as NADH

and FADH2 In photosynthetic organisms the electrons

are donated by water and excited by light energy

The electron donor is oxidized as the electrons are

passed to the first protein complex in the chain As

it accepts the electrons the protein complex is

reduced Next the electrons are passed from the

28

first protein complex which becomes oxidized to

the second protein complex which becomes reduced

Each of these redox reactions is slightly exergonic

or energy releasing Some of this energy is used to

move protons across the membrane against their

concentration gradient With each successive

transfer of electrons the original excited

electrons lose some of their energy After several

successive redox reactions the electrons are

donated to a terminal electron acceptor In aerobic

respiration the terminal electron acceptor is

oxygen In photosynthesis the terminal electron

acceptor is a compound called NADP

The energy in the excited electrons has been

transformed into an electrochemical gradient

represented by the high concentration of protons on

one side of the membrane These protons can diffuse

back across the membrane by passing through a

protein complex called ATP synthase As these

protons pass through ATP synthase down their

concentration gradient they provide the energy

needed to make ATP from ADP and inorganic phosphate

within the cell

29

CHAPTER III

CLOSING

A Conclution

When a hexose is completely oxidized to CO2

and H2O using these three processes Glycolysis

yields two ATP and two NADH per hexose used Each

such NADH oxidized by the electron transport system

30

yields two ATP as dscribed above so glycolysis

contributes a total of six ATP per hexose The Krebs

cycle contributes two ATP per hexose or per two

pyruvates when succinyl CoA is cleaved to succinate

and CoASH This cycle also produces eight NADH per

hexose within the mitochondrial matrix by oxidative

phosporylation each of these NADH yields three ATP

or 24 per hexose Each FADH2 from the Krebs cycle

yields two ATP by oxydative phosporylation or four

per hexose (two pyruvates) The total contribution of

the Krebs cycle is then 30 Adding these 30 to the 6

from glycolysis leads to a total of 36 per hexose

completely respired by these process

B Suggestion

My greatest wish is that the readers find

this manuscript helpful in gaining the knowledge they

need for effective learning and practice and in

preparing for examination designed to test that

knowledge Reader comments and suggestions are

welcome and would be very helpful in improving our

manuscript Your contribution may assist many other

students in the coming years

31

REFERENCE

Anonim 2013 Anaerobic Respiration

httpwwwscoolcoukgcsebiologyrespiration

revise-itanaerobic-respiration Accessed

September 10th 2013

Anonim 2013 What is the difference between aerobic

and anaerobic respiration in plants

httpwwwbiologylifeeasyorg4010difference

-between-aerobic-anaerobic-respiration-plants

Accessed September 10th 2013

Campbell Mary K 2006 Biochemistry New York Thomson

Brooks Cole Inc

Dat J Capelli N Folzer H Bourgeade P Badot P-M

(2004) Sensing and signaling during

plantflooding Plant Physiology and

Biochemistry 42 273-282

Dat J Folzer H Parent C Badot P-M Capelli N

(2006) HypoxiastressCurrent Understanding and

Perspectives In Teixeira da Silva JA (Ed)

Floriculture Ornamental and Plant

Biotechnology Advances and Topical Issues (Vol

3) Global Science Books Isleworth United

Kingdompp 664-674

Although higher plants are obligate aerobs and

are able to tolerate anoxia for only short periods

tissues or organs are occasionally subjected to

anaerobic conditions A typical siuation is that of

roots when the soil is saturated with water When

there is no oxygen to serve as the terminal electron

acceptor mitochondrial respiration will shut down

and metabolism will shift over to fermentation

Fermentation converts pyruvate to either ethanol

through the action of the enzyme alcohol

dehydrogenase (ADH) or lactate via lactate

dehydrogenase (LDH) In most plants the principal

products of fermentation are CO2 and ethanol Some

lactate may be formed primarily in the early stages

of anoxia However lactate lowers he pH of the

cytosol which in turn activates pyruvate

decarboxylase and initiates the production of

ethanol

Either one of the fermentation reactions

consumes the NADH produced earlier in glycolysis by

the oxidation of glyceraldehide-3-P Although this

means there is no net gain of reducing potential in

fermentation This recycling of NADH is still

important to the cell The poolof NADH plus NAD+ in

the cell is relatively small and if the NADH is not

recycled there will be no supply of NAD+ to support

14

the continued oxidation of glyceraldehyde-3-P if

this were the case glycolysis and the producton of

even the small quantities of ATP necessary to

maintain the cells under anaerobic conditions would

then grind to a halt (Hopkins 2009 241)

Soil waterlogging has long been identified as a

major abiotic stress and the constraints that it

provides the roots have a significant effect on the

growth and development of plants When this event

occurs in the spring then these puddles can reduce

seed germination and seedling development Thus a

pool of water is an important factor that affects

the growth development and survival of plant

species not only in natural ecosystems but also

the agricultural and horticultural systems (Dat et

al 2006)

After flooding rapid changes in soil

properties At the time of pore water meets the

land the air pushed out gas diffusion is reduced

and toxic compounds accumulate due to anaerobic

conditions All of these changes greatly affect the

ability of plants to survive In response increased

stomatal resistance photosynthesis and root

hydraulic conductivity decreases and reduced

translocation fotoassimilat Nevertheless one of

15

the best adaptations of plants to hypoxia anoxia

is transitional biochemical and metabolic processes

common to current limited availability of O2 (Dat et

al 2004)

Selective synthesis of a set of about 20

anaerobic stress proteins (ANPS) allows the process

of producing energy without oxygen metabolism under

anaerobic conditions (Subbaiah and Sachs 2003)

Another adaptation is the observed morphological

changes consisting of the formation of hypertrophic

lenticels adventitious root initiation and or

progression of aerenchyma (Vartapetian and Jackson

1997 Jackson and Colmer 2005 Folzer et al 2006)

This review details the diverse plant stress

response to hypoxia anoxia caused by soil

waterlogging flooding and examines some of the key

features of metabolic adaptation physiological and

morphological

16

C Electron

Transport System

We noted earlier that one of the principal

functions of the respiration is to retrieve in

useful form some of the energy initially stored in

assimilates Our traditional measure of useful

energy in most processes is the number of ATP

molecules gained or consumed By this measure alone

the yield from both glycolysis and the citric acid

cycle is quite low After two complete turns of the

cycle one molecule of glucose has been completely

oxidized to six molecules of CO2 but only four

molecules of ATP have been produced (a net of two

ATP from glycolysis plus one for each turn of the

cycle) At this point most of the energy associated

with the glucose molecule has been conserved in the

17

form of electron pairs generated by the oxidation of

glycolytic and citric acid cycle intermediates

In this section we will discuss the third stage

of cellular respirationmdashthe transfer of electrons

fromNADH and FADH2 to oxygen and the accompanying

conversion of redox energy to ATP The transfer of

electrons fromNADHand FADH2 to oxygen involves a

sequence of electron carriers arranged in an

electron transport chainMembrane fractionation

studies have shown that the enzymes and electron

carriers making up the electron transport chain are

organized predominantly into four large

multimolecular complexes (complexes ImdashIV) and two

mobile carriers located in the inner mitochondrial

membrane (Figure 109) In this sense there are a

great number of similarities between the

mitochondrial inner membrane and the thylakoid

membranes of the chloroplast (compare Figure 109

with Figure 76) This is not unexpected since the

principal function of each membrane is energy

transformation and many of the same or similar

components are involved

The Components of the Electron Transport Chain

The electron transport chain of the

mitochondria is the means by which electrons are

18

removed from the reduced carrier NADH and

transferred to oxygen to yield H2O

1) NADH

NADH is generated in the matrix by the reactions

of pyruvate dehydrogenase isocitrate

dehydrogenase α-ketoglutarate dehydrogenase and

malate dehyrogenase The electron transport chain

begins with reoxidizing NADH to form NAD+ and

channeling the electrons into the formation of

reduced coenzymes Important to note that NADH

transfers 2 electrons at a time in the form of a

hydride

NAD+ + 2e- + H+ 1048581 NADH Eorsquo = minus0315 V

2) Flavoproteins

Flavoproteins have either a FAD (flavin adenosine

dinucleotide) or a FMN (flavin mononucleotide)

prosthetic group Flavoproteins can accept or

donate electrons one at time or two at a time

Thus they are often intermediaries between two

electron acceptorsdonors and one electron

acceptorsdonors For flavoproteins the typical

standard reduction potentials are around 0 V

FAD + 2e- + 2H+ 1048581 FADH2 FMN + 2e- + 2H+ 1048581 FMNH2

Eorsquo asymp 0 V

19

3) Coenzyme Q (CoQ)

aka ubiquinone (UQ) Shown below CoQ has ten

repeating isoprene units which make it

insoluble in water but soluble in the

hydrophobic lipid bylayer Coenzyme Q is a

versatile cofactor because it is a soluble

electron carrier in the hydrophobic bilipid layer

of the

inner mitochondrial membrane Like flavoproteins

CoQ can acceptdonate electrons one at a time or

two at a time

Q + 2e- + 2H+ 1048581 QH2 Eorsquo =0060 V

Q + e- + H+ 1048581 1048581QH Eorsquo =0030 V

1048581QH + e- + H+ 1048581 QH2 Eorsquo =0190 V

4) Cytochromes

Cytochromes are proteins that contain heme

prosthetic groups which function as

one electron carriers The heme iron is involved

in one electron transfers involving the Fe2+ and

Fe3+ oxidation states Cytochromes are named by

their absorption spectra which depends

on the porphyrin structure and environment The

example shown is the heme prosthetic group of

cytochrome b Cytochrome b contains the same iron

porphyrin found in hemoglobin and myoglobin

20

Other cytochromes we will encounter in the

electron transport complexes are cytochromes b

c c1 a and a3

5) Iron-Sulfur Proteins

In the electron transport chain we will encounter

many iron-sulfer proteins which participate in

one electron transfers involving the the Fe2+ and

Fe3+ oxidation states These are non-heme iron-

sulfur proteinsThe simplest iron-sulfer protein

is FeS in which iron is tetrahedrally coordinated

by four cysteines The second form is Fe2S2 which

contains two irons complexed to 2 cysteine

residues and two inorganic sulfides The third

form is Fe3S4 which contains 3 iron atoms

coordinated to three cysteine residues and 4

inorganic sulfides The last form is the most

complicated Fe4S4 which contains 4 iron atoms

coordinated to 4 cysteine residues and 4

inorganic sulfides

6) Copper Proteins

Copper bound proteins participate in one electron

transfers involving the Cu+ and Cu2+ oxidation

states

21

Overview of the Electron Transport Chain

Electrons move along the electron transport

chain going from donor to acceptor until they reach

oxygen the ultimate electron acceptor The standard

reduction potentials of the electron carriers are

between the NADHNAD+ couple (-0315 V) and the

oxygenH2O couple (0816 V) as on the next page The

components of the electron transport chain are

organized into 4 complexes Each complex contains

several different electron carriers

1 Complex I also known as the NADH-coenzyme Q

reductase or NADH dehydrogenase

2 Complex II also known as succinate-coenzyme Q

reductase or succinate dehydrogenase

3 Complex III also known as coenzyme Q reductase

4 Complex IV also known as cytochrome c reductase

The free energy necessary to generate ATP

is extracted from the oxidation of NADH and

FADH2 by the electron transport chain It

consists of four protein complexes and single

protein chain of cytochrome c through which

electrons pass from lower to higher standard

redox potentials Electrons are carried from complex

I and II to complex III by the membrane (lipid)

soluble coenzyme Q (CoQ or ubiquinone) and between

22

complexes III and IV by the peripheral membrane

protein cytochrome c

In the following I give the overall

reactions catalysed by corresponding protein

complexes in the mitochondrial electron transport

chain

1 Complex I

Complex I is also called NADH-Coenzyme Q

reductase because this large protein complex

transfers 2 electrons from NADH to coenzyme Q

Complex I was formerly known as NADH dehydrogenase

Complex I is huge 850000 kD and is composed of more

than thirty subunits It contains a FMN prosthetic

group and seven or more Fe-S clusters This complex

has between 20-26 iron atoms boundThe prosthetic

group FMN is absolutely required for activity

Therefore this complex is a flavoprotein This

complex binds NADH transfers two electrons in the

form of a hydride to FMN to produce NAD+ and FMNH2

23

The subsequent steps involve the transfer of

electrons one at a time to a series of iron-sulfer

complexes that includes both 2Fe-2S and 4Fe-4S

clusters Note the importance of FMN First it

functions as a 2 electron acceptor in the hydride

transfer from NADH Second it functions as a 1

electron donor to the series of iron sulfur clusters

FMN and FAD often play crucial links between 2

electron transfer agents and 1 electron transfer

agents The final step of this complex is the

transfer of 2 electrons one at a time to coenzyme Q

CoQ like FMN and FAD can function as a 2 electron

donoracceptor and as a 1 electron donoracceptor

CoQ is a mobile electron carrier because its

isoprenoid tail makes it highly hydrophobic and

lipophillic It diffuses freely in the bilipid layer

of the inner mitochondrial membrane

The process of transferring electrons from NADH

to CoQ by complex I results in the net transport of

protons from the matrix side of the inner

mitochondrial membrane to the inter membrane space

where the H+ ions accumulate generating a proton

motive force The intermembrane space side of the

inner membrane is referred to as the P face (P

standing for positive) The matrix side of the inner

membrane is referred to as the the N face The

24

transport of electrons from NADH to CoQ is coupled to

the transport of protons across the membrane This is

an example of active transport The stiochiometry is

4 H+ transported per 2 electrons

2 Complex II

Believe it or not you are already familiar

with Complex II It is none other than succinate

dehydrogenase The only enzyme of the citric acid

cycle that is an integral membrane protein This

complex is composed of four subunits 2 of which are

iron-sulfur proteins and the other two subunits

together bind FAD through a covalent link to a

histidine residue These two subunits are called

flavoprotein 2 or FP2 Complex II contains 3 Fe-S

centers 1 4Fe-4S cluster 1 3Fe-4S cluster and 1

2Fe- 2S cluster In the first step of this complex

succinate is bound and a hydride is transferred to

FAD to generate FADH2 and fumarate FADH2 then

transfers its electrons one at a time to the Fe-S

centers Thus once again FAD functions as 2 electron

acceptor and a 1 electron donor The final step of

this complex is the transfer of 2 electrons one at a

time to coenzyme Q to produce CoQH2 For complex II

the standard free energy change of the overall

reaction is too small to drive the transport of

protons across the inner mitochondrial membrane This

25

accounts for the 15 ATPrsquos generated per FADH2

compared with the 25 ATPrsquos generated per NADH

3 Complex III

This complex is also known as coenzyme Q-

cytochrome c reductase because it passes the

electrons form CoQH2 to cyt c through a very unique

electron transport pathway called the Q-cycle Shown

to the left are the porphyrins found in cytochromes

Cytochrome b contains the same iron protoporphyrin as

hemoglobin and myoglobin The c cytochromes contain

heme c through covalent attachment by cysteine

residues Cytochrome a is found in two forms in

complex IV In complex III we find two b-type

cytochromes and one c-type cytochrome Complex III is

complex and we have a crystal structure

4 Complex IV

Complex IV is also known as cytochrome c oxidase

because it accepts the electrons from cytochrome c

and directs them towards the four electron reduction

of O2 to form 2 molecules of H2O

4 cyt c (Fe2+) + 4 H+ + O2 4 cyt c (Fe3+) +

2H2O

26

Cytochrome c oxidase contains 2 heme

centerscytochrome a and cytochrome a3 and two copper

proteins Each of the protein bound coppers are

associated with one of the cytochromes The copper

sites are called CuA and CuB CuA is associated with

cytochrome a and is shown to the left CuB is

associated with cytochrome a3 The copper sites

function as 1 electron carriers cycling between the

cuprous state Cu+ and the cupric state Cu2+ Just

like iron containing proteins they transfer

electrons one at a time Cytochrome c is bound on the

P-face of the membrane and transfers its electron to

CuA The oxidized cytochrome c dissociates CuA then

transfers the electron to cytochrome a The protein

bound CuA and the iron bound in cytochrome a are 15 Aring

apart In contrast the CuB and the iron bound in

cytochrome a3 are very close to each other forming a

binuclear metal center shown below

27

Electron transport chains are associated with

membranes such as the mitochondrial membrane in

eukaryotic organisms the thylakoid membrane of

photosynthetic organisms and the plasma membrane of

prokaryotic organisms While some of the details

differ electron transport chains in all organisms

operate on the same basic principles

Excited electrons are brought to the electron

transport chain by electron carriers such as NADH

and FADH2 In photosynthetic organisms the electrons

are donated by water and excited by light energy

The electron donor is oxidized as the electrons are

passed to the first protein complex in the chain As

it accepts the electrons the protein complex is

reduced Next the electrons are passed from the

28

first protein complex which becomes oxidized to

the second protein complex which becomes reduced

Each of these redox reactions is slightly exergonic

or energy releasing Some of this energy is used to

move protons across the membrane against their

concentration gradient With each successive

transfer of electrons the original excited

electrons lose some of their energy After several

successive redox reactions the electrons are

donated to a terminal electron acceptor In aerobic

respiration the terminal electron acceptor is

oxygen In photosynthesis the terminal electron

acceptor is a compound called NADP

The energy in the excited electrons has been

transformed into an electrochemical gradient

represented by the high concentration of protons on

one side of the membrane These protons can diffuse

back across the membrane by passing through a

protein complex called ATP synthase As these

protons pass through ATP synthase down their

concentration gradient they provide the energy

needed to make ATP from ADP and inorganic phosphate

within the cell

29

CHAPTER III

CLOSING

A Conclution

When a hexose is completely oxidized to CO2

and H2O using these three processes Glycolysis

yields two ATP and two NADH per hexose used Each

such NADH oxidized by the electron transport system

30

yields two ATP as dscribed above so glycolysis

contributes a total of six ATP per hexose The Krebs

cycle contributes two ATP per hexose or per two

pyruvates when succinyl CoA is cleaved to succinate

and CoASH This cycle also produces eight NADH per

hexose within the mitochondrial matrix by oxidative

phosporylation each of these NADH yields three ATP

or 24 per hexose Each FADH2 from the Krebs cycle

yields two ATP by oxydative phosporylation or four

per hexose (two pyruvates) The total contribution of

the Krebs cycle is then 30 Adding these 30 to the 6

from glycolysis leads to a total of 36 per hexose

completely respired by these process

B Suggestion

My greatest wish is that the readers find

this manuscript helpful in gaining the knowledge they

need for effective learning and practice and in

preparing for examination designed to test that

knowledge Reader comments and suggestions are

welcome and would be very helpful in improving our

manuscript Your contribution may assist many other

students in the coming years

31

REFERENCE

Anonim 2013 Anaerobic Respiration

httpwwwscoolcoukgcsebiologyrespiration

revise-itanaerobic-respiration Accessed

September 10th 2013

Anonim 2013 What is the difference between aerobic

and anaerobic respiration in plants

httpwwwbiologylifeeasyorg4010difference

-between-aerobic-anaerobic-respiration-plants

Accessed September 10th 2013

Campbell Mary K 2006 Biochemistry New York Thomson

Brooks Cole Inc

Dat J Capelli N Folzer H Bourgeade P Badot P-M

(2004) Sensing and signaling during

plantflooding Plant Physiology and

Biochemistry 42 273-282

Dat J Folzer H Parent C Badot P-M Capelli N

(2006) HypoxiastressCurrent Understanding and

Perspectives In Teixeira da Silva JA (Ed)

Floriculture Ornamental and Plant

Biotechnology Advances and Topical Issues (Vol

3) Global Science Books Isleworth United

Kingdompp 664-674

the continued oxidation of glyceraldehyde-3-P if

this were the case glycolysis and the producton of

even the small quantities of ATP necessary to

maintain the cells under anaerobic conditions would

then grind to a halt (Hopkins 2009 241)

Soil waterlogging has long been identified as a

major abiotic stress and the constraints that it

provides the roots have a significant effect on the

growth and development of plants When this event

occurs in the spring then these puddles can reduce

seed germination and seedling development Thus a

pool of water is an important factor that affects

the growth development and survival of plant

species not only in natural ecosystems but also

the agricultural and horticultural systems (Dat et

al 2006)

After flooding rapid changes in soil

properties At the time of pore water meets the

land the air pushed out gas diffusion is reduced

and toxic compounds accumulate due to anaerobic

conditions All of these changes greatly affect the

ability of plants to survive In response increased

stomatal resistance photosynthesis and root

hydraulic conductivity decreases and reduced

translocation fotoassimilat Nevertheless one of

15

the best adaptations of plants to hypoxia anoxia

is transitional biochemical and metabolic processes

common to current limited availability of O2 (Dat et

al 2004)

Selective synthesis of a set of about 20

anaerobic stress proteins (ANPS) allows the process

of producing energy without oxygen metabolism under

anaerobic conditions (Subbaiah and Sachs 2003)

Another adaptation is the observed morphological

changes consisting of the formation of hypertrophic

lenticels adventitious root initiation and or

progression of aerenchyma (Vartapetian and Jackson

1997 Jackson and Colmer 2005 Folzer et al 2006)

This review details the diverse plant stress

response to hypoxia anoxia caused by soil

waterlogging flooding and examines some of the key

features of metabolic adaptation physiological and

morphological

16

C Electron

Transport System

We noted earlier that one of the principal

functions of the respiration is to retrieve in

useful form some of the energy initially stored in

assimilates Our traditional measure of useful

energy in most processes is the number of ATP

molecules gained or consumed By this measure alone

the yield from both glycolysis and the citric acid

cycle is quite low After two complete turns of the

cycle one molecule of glucose has been completely

oxidized to six molecules of CO2 but only four

molecules of ATP have been produced (a net of two

ATP from glycolysis plus one for each turn of the

cycle) At this point most of the energy associated

with the glucose molecule has been conserved in the

17

form of electron pairs generated by the oxidation of

glycolytic and citric acid cycle intermediates

In this section we will discuss the third stage

of cellular respirationmdashthe transfer of electrons

fromNADH and FADH2 to oxygen and the accompanying

conversion of redox energy to ATP The transfer of

electrons fromNADHand FADH2 to oxygen involves a

sequence of electron carriers arranged in an

electron transport chainMembrane fractionation

studies have shown that the enzymes and electron

carriers making up the electron transport chain are

organized predominantly into four large

multimolecular complexes (complexes ImdashIV) and two

mobile carriers located in the inner mitochondrial

membrane (Figure 109) In this sense there are a

great number of similarities between the

mitochondrial inner membrane and the thylakoid

membranes of the chloroplast (compare Figure 109

with Figure 76) This is not unexpected since the

principal function of each membrane is energy

transformation and many of the same or similar

components are involved

The Components of the Electron Transport Chain

The electron transport chain of the

mitochondria is the means by which electrons are

18

removed from the reduced carrier NADH and

transferred to oxygen to yield H2O

1) NADH

NADH is generated in the matrix by the reactions

of pyruvate dehydrogenase isocitrate

dehydrogenase α-ketoglutarate dehydrogenase and

malate dehyrogenase The electron transport chain

begins with reoxidizing NADH to form NAD+ and

channeling the electrons into the formation of

reduced coenzymes Important to note that NADH

transfers 2 electrons at a time in the form of a

hydride

NAD+ + 2e- + H+ 1048581 NADH Eorsquo = minus0315 V

2) Flavoproteins

Flavoproteins have either a FAD (flavin adenosine

dinucleotide) or a FMN (flavin mononucleotide)

prosthetic group Flavoproteins can accept or

donate electrons one at time or two at a time

Thus they are often intermediaries between two

electron acceptorsdonors and one electron

acceptorsdonors For flavoproteins the typical

standard reduction potentials are around 0 V

FAD + 2e- + 2H+ 1048581 FADH2 FMN + 2e- + 2H+ 1048581 FMNH2

Eorsquo asymp 0 V

19

3) Coenzyme Q (CoQ)

aka ubiquinone (UQ) Shown below CoQ has ten

repeating isoprene units which make it

insoluble in water but soluble in the

hydrophobic lipid bylayer Coenzyme Q is a

versatile cofactor because it is a soluble

electron carrier in the hydrophobic bilipid layer

of the

inner mitochondrial membrane Like flavoproteins

CoQ can acceptdonate electrons one at a time or

two at a time

Q + 2e- + 2H+ 1048581 QH2 Eorsquo =0060 V

Q + e- + H+ 1048581 1048581QH Eorsquo =0030 V

1048581QH + e- + H+ 1048581 QH2 Eorsquo =0190 V

4) Cytochromes

Cytochromes are proteins that contain heme

prosthetic groups which function as

one electron carriers The heme iron is involved

in one electron transfers involving the Fe2+ and

Fe3+ oxidation states Cytochromes are named by

their absorption spectra which depends

on the porphyrin structure and environment The

example shown is the heme prosthetic group of

cytochrome b Cytochrome b contains the same iron

porphyrin found in hemoglobin and myoglobin

20

Other cytochromes we will encounter in the

electron transport complexes are cytochromes b

c c1 a and a3

5) Iron-Sulfur Proteins

In the electron transport chain we will encounter

many iron-sulfer proteins which participate in

one electron transfers involving the the Fe2+ and

Fe3+ oxidation states These are non-heme iron-

sulfur proteinsThe simplest iron-sulfer protein

is FeS in which iron is tetrahedrally coordinated

by four cysteines The second form is Fe2S2 which

contains two irons complexed to 2 cysteine

residues and two inorganic sulfides The third

form is Fe3S4 which contains 3 iron atoms

coordinated to three cysteine residues and 4

inorganic sulfides The last form is the most

complicated Fe4S4 which contains 4 iron atoms

coordinated to 4 cysteine residues and 4

inorganic sulfides

6) Copper Proteins

Copper bound proteins participate in one electron

transfers involving the Cu+ and Cu2+ oxidation

states

21

Overview of the Electron Transport Chain

Electrons move along the electron transport

chain going from donor to acceptor until they reach

oxygen the ultimate electron acceptor The standard

reduction potentials of the electron carriers are

between the NADHNAD+ couple (-0315 V) and the

oxygenH2O couple (0816 V) as on the next page The

components of the electron transport chain are

organized into 4 complexes Each complex contains

several different electron carriers

1 Complex I also known as the NADH-coenzyme Q

reductase or NADH dehydrogenase

2 Complex II also known as succinate-coenzyme Q

reductase or succinate dehydrogenase

3 Complex III also known as coenzyme Q reductase

4 Complex IV also known as cytochrome c reductase

The free energy necessary to generate ATP

is extracted from the oxidation of NADH and

FADH2 by the electron transport chain It

consists of four protein complexes and single

protein chain of cytochrome c through which

electrons pass from lower to higher standard

redox potentials Electrons are carried from complex

I and II to complex III by the membrane (lipid)

soluble coenzyme Q (CoQ or ubiquinone) and between

22

complexes III and IV by the peripheral membrane

protein cytochrome c

In the following I give the overall

reactions catalysed by corresponding protein

complexes in the mitochondrial electron transport

chain

1 Complex I

Complex I is also called NADH-Coenzyme Q

reductase because this large protein complex

transfers 2 electrons from NADH to coenzyme Q

Complex I was formerly known as NADH dehydrogenase

Complex I is huge 850000 kD and is composed of more

than thirty subunits It contains a FMN prosthetic

group and seven or more Fe-S clusters This complex

has between 20-26 iron atoms boundThe prosthetic

group FMN is absolutely required for activity

Therefore this complex is a flavoprotein This

complex binds NADH transfers two electrons in the

form of a hydride to FMN to produce NAD+ and FMNH2

23

The subsequent steps involve the transfer of

electrons one at a time to a series of iron-sulfer

complexes that includes both 2Fe-2S and 4Fe-4S

clusters Note the importance of FMN First it

functions as a 2 electron acceptor in the hydride

transfer from NADH Second it functions as a 1

electron donor to the series of iron sulfur clusters

FMN and FAD often play crucial links between 2

electron transfer agents and 1 electron transfer

agents The final step of this complex is the

transfer of 2 electrons one at a time to coenzyme Q

CoQ like FMN and FAD can function as a 2 electron

donoracceptor and as a 1 electron donoracceptor

CoQ is a mobile electron carrier because its

isoprenoid tail makes it highly hydrophobic and

lipophillic It diffuses freely in the bilipid layer

of the inner mitochondrial membrane

The process of transferring electrons from NADH

to CoQ by complex I results in the net transport of

protons from the matrix side of the inner

mitochondrial membrane to the inter membrane space

where the H+ ions accumulate generating a proton

motive force The intermembrane space side of the

inner membrane is referred to as the P face (P

standing for positive) The matrix side of the inner

membrane is referred to as the the N face The

24

transport of electrons from NADH to CoQ is coupled to

the transport of protons across the membrane This is

an example of active transport The stiochiometry is

4 H+ transported per 2 electrons

2 Complex II

Believe it or not you are already familiar

with Complex II It is none other than succinate

dehydrogenase The only enzyme of the citric acid

cycle that is an integral membrane protein This

complex is composed of four subunits 2 of which are

iron-sulfur proteins and the other two subunits

together bind FAD through a covalent link to a

histidine residue These two subunits are called

flavoprotein 2 or FP2 Complex II contains 3 Fe-S

centers 1 4Fe-4S cluster 1 3Fe-4S cluster and 1

2Fe- 2S cluster In the first step of this complex

succinate is bound and a hydride is transferred to

FAD to generate FADH2 and fumarate FADH2 then

transfers its electrons one at a time to the Fe-S

centers Thus once again FAD functions as 2 electron

acceptor and a 1 electron donor The final step of

this complex is the transfer of 2 electrons one at a

time to coenzyme Q to produce CoQH2 For complex II

the standard free energy change of the overall

reaction is too small to drive the transport of

protons across the inner mitochondrial membrane This

25

accounts for the 15 ATPrsquos generated per FADH2

compared with the 25 ATPrsquos generated per NADH

3 Complex III

This complex is also known as coenzyme Q-

cytochrome c reductase because it passes the

electrons form CoQH2 to cyt c through a very unique

electron transport pathway called the Q-cycle Shown

to the left are the porphyrins found in cytochromes

Cytochrome b contains the same iron protoporphyrin as

hemoglobin and myoglobin The c cytochromes contain

heme c through covalent attachment by cysteine

residues Cytochrome a is found in two forms in

complex IV In complex III we find two b-type

cytochromes and one c-type cytochrome Complex III is

complex and we have a crystal structure

4 Complex IV

Complex IV is also known as cytochrome c oxidase

because it accepts the electrons from cytochrome c

and directs them towards the four electron reduction

of O2 to form 2 molecules of H2O

4 cyt c (Fe2+) + 4 H+ + O2 4 cyt c (Fe3+) +

2H2O

26

Cytochrome c oxidase contains 2 heme

centerscytochrome a and cytochrome a3 and two copper

proteins Each of the protein bound coppers are

associated with one of the cytochromes The copper

sites are called CuA and CuB CuA is associated with

cytochrome a and is shown to the left CuB is

associated with cytochrome a3 The copper sites

function as 1 electron carriers cycling between the

cuprous state Cu+ and the cupric state Cu2+ Just

like iron containing proteins they transfer

electrons one at a time Cytochrome c is bound on the

P-face of the membrane and transfers its electron to

CuA The oxidized cytochrome c dissociates CuA then

transfers the electron to cytochrome a The protein

bound CuA and the iron bound in cytochrome a are 15 Aring

apart In contrast the CuB and the iron bound in

cytochrome a3 are very close to each other forming a

binuclear metal center shown below

27

Electron transport chains are associated with

membranes such as the mitochondrial membrane in

eukaryotic organisms the thylakoid membrane of

photosynthetic organisms and the plasma membrane of

prokaryotic organisms While some of the details

differ electron transport chains in all organisms

operate on the same basic principles

Excited electrons are brought to the electron

transport chain by electron carriers such as NADH

and FADH2 In photosynthetic organisms the electrons

are donated by water and excited by light energy

The electron donor is oxidized as the electrons are

passed to the first protein complex in the chain As

it accepts the electrons the protein complex is

reduced Next the electrons are passed from the

28

first protein complex which becomes oxidized to

the second protein complex which becomes reduced

Each of these redox reactions is slightly exergonic

or energy releasing Some of this energy is used to

move protons across the membrane against their

concentration gradient With each successive

transfer of electrons the original excited

electrons lose some of their energy After several

successive redox reactions the electrons are

donated to a terminal electron acceptor In aerobic

respiration the terminal electron acceptor is

oxygen In photosynthesis the terminal electron

acceptor is a compound called NADP

The energy in the excited electrons has been

transformed into an electrochemical gradient

represented by the high concentration of protons on

one side of the membrane These protons can diffuse

back across the membrane by passing through a

protein complex called ATP synthase As these

protons pass through ATP synthase down their

concentration gradient they provide the energy

needed to make ATP from ADP and inorganic phosphate

within the cell

29

CHAPTER III

CLOSING

A Conclution

When a hexose is completely oxidized to CO2

and H2O using these three processes Glycolysis

yields two ATP and two NADH per hexose used Each

such NADH oxidized by the electron transport system

30

yields two ATP as dscribed above so glycolysis

contributes a total of six ATP per hexose The Krebs

cycle contributes two ATP per hexose or per two

pyruvates when succinyl CoA is cleaved to succinate

and CoASH This cycle also produces eight NADH per

hexose within the mitochondrial matrix by oxidative

phosporylation each of these NADH yields three ATP

or 24 per hexose Each FADH2 from the Krebs cycle

yields two ATP by oxydative phosporylation or four

per hexose (two pyruvates) The total contribution of

the Krebs cycle is then 30 Adding these 30 to the 6

from glycolysis leads to a total of 36 per hexose

completely respired by these process

B Suggestion

My greatest wish is that the readers find

this manuscript helpful in gaining the knowledge they

need for effective learning and practice and in

preparing for examination designed to test that

knowledge Reader comments and suggestions are

welcome and would be very helpful in improving our

manuscript Your contribution may assist many other

students in the coming years

31

REFERENCE

Anonim 2013 Anaerobic Respiration

httpwwwscoolcoukgcsebiologyrespiration

revise-itanaerobic-respiration Accessed

September 10th 2013

Anonim 2013 What is the difference between aerobic

and anaerobic respiration in plants

httpwwwbiologylifeeasyorg4010difference

-between-aerobic-anaerobic-respiration-plants

Accessed September 10th 2013

Campbell Mary K 2006 Biochemistry New York Thomson

Brooks Cole Inc

Dat J Capelli N Folzer H Bourgeade P Badot P-M

(2004) Sensing and signaling during

plantflooding Plant Physiology and

Biochemistry 42 273-282

Dat J Folzer H Parent C Badot P-M Capelli N

(2006) HypoxiastressCurrent Understanding and

Perspectives In Teixeira da Silva JA (Ed)

Floriculture Ornamental and Plant

Biotechnology Advances and Topical Issues (Vol

3) Global Science Books Isleworth United

Kingdompp 664-674

the best adaptations of plants to hypoxia anoxia

is transitional biochemical and metabolic processes

common to current limited availability of O2 (Dat et

al 2004)

Selective synthesis of a set of about 20

anaerobic stress proteins (ANPS) allows the process

of producing energy without oxygen metabolism under

anaerobic conditions (Subbaiah and Sachs 2003)

Another adaptation is the observed morphological

changes consisting of the formation of hypertrophic

lenticels adventitious root initiation and or

progression of aerenchyma (Vartapetian and Jackson

1997 Jackson and Colmer 2005 Folzer et al 2006)

This review details the diverse plant stress

response to hypoxia anoxia caused by soil

waterlogging flooding and examines some of the key

features of metabolic adaptation physiological and

morphological

16

C Electron

Transport System

We noted earlier that one of the principal

functions of the respiration is to retrieve in

useful form some of the energy initially stored in

assimilates Our traditional measure of useful

energy in most processes is the number of ATP

molecules gained or consumed By this measure alone

the yield from both glycolysis and the citric acid

cycle is quite low After two complete turns of the

cycle one molecule of glucose has been completely

oxidized to six molecules of CO2 but only four

molecules of ATP have been produced (a net of two

ATP from glycolysis plus one for each turn of the

cycle) At this point most of the energy associated

with the glucose molecule has been conserved in the

17

form of electron pairs generated by the oxidation of

glycolytic and citric acid cycle intermediates

In this section we will discuss the third stage

of cellular respirationmdashthe transfer of electrons

fromNADH and FADH2 to oxygen and the accompanying

conversion of redox energy to ATP The transfer of

electrons fromNADHand FADH2 to oxygen involves a

sequence of electron carriers arranged in an

electron transport chainMembrane fractionation

studies have shown that the enzymes and electron

carriers making up the electron transport chain are

organized predominantly into four large

multimolecular complexes (complexes ImdashIV) and two

mobile carriers located in the inner mitochondrial

membrane (Figure 109) In this sense there are a

great number of similarities between the

mitochondrial inner membrane and the thylakoid

membranes of the chloroplast (compare Figure 109

with Figure 76) This is not unexpected since the

principal function of each membrane is energy

transformation and many of the same or similar

components are involved

The Components of the Electron Transport Chain

The electron transport chain of the

mitochondria is the means by which electrons are

18

removed from the reduced carrier NADH and

transferred to oxygen to yield H2O

1) NADH

NADH is generated in the matrix by the reactions

of pyruvate dehydrogenase isocitrate

dehydrogenase α-ketoglutarate dehydrogenase and

malate dehyrogenase The electron transport chain

begins with reoxidizing NADH to form NAD+ and

channeling the electrons into the formation of

reduced coenzymes Important to note that NADH

transfers 2 electrons at a time in the form of a

hydride

NAD+ + 2e- + H+ 1048581 NADH Eorsquo = minus0315 V

2) Flavoproteins

Flavoproteins have either a FAD (flavin adenosine

dinucleotide) or a FMN (flavin mononucleotide)

prosthetic group Flavoproteins can accept or

donate electrons one at time or two at a time

Thus they are often intermediaries between two

electron acceptorsdonors and one electron

acceptorsdonors For flavoproteins the typical

standard reduction potentials are around 0 V

FAD + 2e- + 2H+ 1048581 FADH2 FMN + 2e- + 2H+ 1048581 FMNH2

Eorsquo asymp 0 V

19

3) Coenzyme Q (CoQ)

aka ubiquinone (UQ) Shown below CoQ has ten

repeating isoprene units which make it

insoluble in water but soluble in the

hydrophobic lipid bylayer Coenzyme Q is a

versatile cofactor because it is a soluble

electron carrier in the hydrophobic bilipid layer

of the

inner mitochondrial membrane Like flavoproteins

CoQ can acceptdonate electrons one at a time or

two at a time

Q + 2e- + 2H+ 1048581 QH2 Eorsquo =0060 V

Q + e- + H+ 1048581 1048581QH Eorsquo =0030 V

1048581QH + e- + H+ 1048581 QH2 Eorsquo =0190 V

4) Cytochromes

Cytochromes are proteins that contain heme

prosthetic groups which function as

one electron carriers The heme iron is involved

in one electron transfers involving the Fe2+ and

Fe3+ oxidation states Cytochromes are named by

their absorption spectra which depends

on the porphyrin structure and environment The

example shown is the heme prosthetic group of

cytochrome b Cytochrome b contains the same iron

porphyrin found in hemoglobin and myoglobin

20

Other cytochromes we will encounter in the

electron transport complexes are cytochromes b

c c1 a and a3

5) Iron-Sulfur Proteins

In the electron transport chain we will encounter

many iron-sulfer proteins which participate in

one electron transfers involving the the Fe2+ and

Fe3+ oxidation states These are non-heme iron-

sulfur proteinsThe simplest iron-sulfer protein

is FeS in which iron is tetrahedrally coordinated

by four cysteines The second form is Fe2S2 which

contains two irons complexed to 2 cysteine

residues and two inorganic sulfides The third

form is Fe3S4 which contains 3 iron atoms

coordinated to three cysteine residues and 4

inorganic sulfides The last form is the most

complicated Fe4S4 which contains 4 iron atoms

coordinated to 4 cysteine residues and 4

inorganic sulfides

6) Copper Proteins

Copper bound proteins participate in one electron

transfers involving the Cu+ and Cu2+ oxidation

states

21

Overview of the Electron Transport Chain

Electrons move along the electron transport

chain going from donor to acceptor until they reach

oxygen the ultimate electron acceptor The standard

reduction potentials of the electron carriers are

between the NADHNAD+ couple (-0315 V) and the

oxygenH2O couple (0816 V) as on the next page The

components of the electron transport chain are

organized into 4 complexes Each complex contains

several different electron carriers

1 Complex I also known as the NADH-coenzyme Q

reductase or NADH dehydrogenase

2 Complex II also known as succinate-coenzyme Q

reductase or succinate dehydrogenase

3 Complex III also known as coenzyme Q reductase

4 Complex IV also known as cytochrome c reductase

The free energy necessary to generate ATP

is extracted from the oxidation of NADH and

FADH2 by the electron transport chain It

consists of four protein complexes and single

protein chain of cytochrome c through which

electrons pass from lower to higher standard

redox potentials Electrons are carried from complex

I and II to complex III by the membrane (lipid)

soluble coenzyme Q (CoQ or ubiquinone) and between

22

complexes III and IV by the peripheral membrane

protein cytochrome c

In the following I give the overall

reactions catalysed by corresponding protein

complexes in the mitochondrial electron transport

chain

1 Complex I

Complex I is also called NADH-Coenzyme Q

reductase because this large protein complex

transfers 2 electrons from NADH to coenzyme Q

Complex I was formerly known as NADH dehydrogenase

Complex I is huge 850000 kD and is composed of more

than thirty subunits It contains a FMN prosthetic

group and seven or more Fe-S clusters This complex

has between 20-26 iron atoms boundThe prosthetic

group FMN is absolutely required for activity

Therefore this complex is a flavoprotein This

complex binds NADH transfers two electrons in the

form of a hydride to FMN to produce NAD+ and FMNH2

23

The subsequent steps involve the transfer of

electrons one at a time to a series of iron-sulfer

complexes that includes both 2Fe-2S and 4Fe-4S

clusters Note the importance of FMN First it

functions as a 2 electron acceptor in the hydride

transfer from NADH Second it functions as a 1

electron donor to the series of iron sulfur clusters

FMN and FAD often play crucial links between 2

electron transfer agents and 1 electron transfer

agents The final step of this complex is the

transfer of 2 electrons one at a time to coenzyme Q

CoQ like FMN and FAD can function as a 2 electron

donoracceptor and as a 1 electron donoracceptor

CoQ is a mobile electron carrier because its

isoprenoid tail makes it highly hydrophobic and

lipophillic It diffuses freely in the bilipid layer

of the inner mitochondrial membrane

The process of transferring electrons from NADH

to CoQ by complex I results in the net transport of

protons from the matrix side of the inner

mitochondrial membrane to the inter membrane space

where the H+ ions accumulate generating a proton

motive force The intermembrane space side of the

inner membrane is referred to as the P face (P

standing for positive) The matrix side of the inner

membrane is referred to as the the N face The

24

transport of electrons from NADH to CoQ is coupled to

the transport of protons across the membrane This is

an example of active transport The stiochiometry is

4 H+ transported per 2 electrons

2 Complex II

Believe it or not you are already familiar

with Complex II It is none other than succinate

dehydrogenase The only enzyme of the citric acid

cycle that is an integral membrane protein This

complex is composed of four subunits 2 of which are

iron-sulfur proteins and the other two subunits

together bind FAD through a covalent link to a

histidine residue These two subunits are called

flavoprotein 2 or FP2 Complex II contains 3 Fe-S

centers 1 4Fe-4S cluster 1 3Fe-4S cluster and 1

2Fe- 2S cluster In the first step of this complex

succinate is bound and a hydride is transferred to

FAD to generate FADH2 and fumarate FADH2 then

transfers its electrons one at a time to the Fe-S

centers Thus once again FAD functions as 2 electron

acceptor and a 1 electron donor The final step of

this complex is the transfer of 2 electrons one at a

time to coenzyme Q to produce CoQH2 For complex II

the standard free energy change of the overall

reaction is too small to drive the transport of

protons across the inner mitochondrial membrane This

25

accounts for the 15 ATPrsquos generated per FADH2

compared with the 25 ATPrsquos generated per NADH

3 Complex III

This complex is also known as coenzyme Q-

cytochrome c reductase because it passes the

electrons form CoQH2 to cyt c through a very unique

electron transport pathway called the Q-cycle Shown

to the left are the porphyrins found in cytochromes

Cytochrome b contains the same iron protoporphyrin as

hemoglobin and myoglobin The c cytochromes contain

heme c through covalent attachment by cysteine

residues Cytochrome a is found in two forms in

complex IV In complex III we find two b-type

cytochromes and one c-type cytochrome Complex III is

complex and we have a crystal structure

4 Complex IV

Complex IV is also known as cytochrome c oxidase

because it accepts the electrons from cytochrome c

and directs them towards the four electron reduction

of O2 to form 2 molecules of H2O

4 cyt c (Fe2+) + 4 H+ + O2 4 cyt c (Fe3+) +

2H2O

26

Cytochrome c oxidase contains 2 heme

centerscytochrome a and cytochrome a3 and two copper

proteins Each of the protein bound coppers are

associated with one of the cytochromes The copper

sites are called CuA and CuB CuA is associated with

cytochrome a and is shown to the left CuB is

associated with cytochrome a3 The copper sites

function as 1 electron carriers cycling between the

cuprous state Cu+ and the cupric state Cu2+ Just

like iron containing proteins they transfer

electrons one at a time Cytochrome c is bound on the

P-face of the membrane and transfers its electron to

CuA The oxidized cytochrome c dissociates CuA then

transfers the electron to cytochrome a The protein

bound CuA and the iron bound in cytochrome a are 15 Aring

apart In contrast the CuB and the iron bound in

cytochrome a3 are very close to each other forming a

binuclear metal center shown below

27

Electron transport chains are associated with

membranes such as the mitochondrial membrane in

eukaryotic organisms the thylakoid membrane of

photosynthetic organisms and the plasma membrane of

prokaryotic organisms While some of the details

differ electron transport chains in all organisms

operate on the same basic principles

Excited electrons are brought to the electron

transport chain by electron carriers such as NADH

and FADH2 In photosynthetic organisms the electrons

are donated by water and excited by light energy

The electron donor is oxidized as the electrons are

passed to the first protein complex in the chain As

it accepts the electrons the protein complex is

reduced Next the electrons are passed from the

28

first protein complex which becomes oxidized to

the second protein complex which becomes reduced

Each of these redox reactions is slightly exergonic

or energy releasing Some of this energy is used to

move protons across the membrane against their

concentration gradient With each successive

transfer of electrons the original excited

electrons lose some of their energy After several

successive redox reactions the electrons are

donated to a terminal electron acceptor In aerobic

respiration the terminal electron acceptor is

oxygen In photosynthesis the terminal electron

acceptor is a compound called NADP

The energy in the excited electrons has been

transformed into an electrochemical gradient

represented by the high concentration of protons on

one side of the membrane These protons can diffuse

back across the membrane by passing through a

protein complex called ATP synthase As these

protons pass through ATP synthase down their

concentration gradient they provide the energy

needed to make ATP from ADP and inorganic phosphate

within the cell

29

CHAPTER III

CLOSING

A Conclution

When a hexose is completely oxidized to CO2

and H2O using these three processes Glycolysis

yields two ATP and two NADH per hexose used Each

such NADH oxidized by the electron transport system

30

yields two ATP as dscribed above so glycolysis

contributes a total of six ATP per hexose The Krebs

cycle contributes two ATP per hexose or per two

pyruvates when succinyl CoA is cleaved to succinate

and CoASH This cycle also produces eight NADH per

hexose within the mitochondrial matrix by oxidative

phosporylation each of these NADH yields three ATP

or 24 per hexose Each FADH2 from the Krebs cycle

yields two ATP by oxydative phosporylation or four

per hexose (two pyruvates) The total contribution of

the Krebs cycle is then 30 Adding these 30 to the 6

from glycolysis leads to a total of 36 per hexose

completely respired by these process

B Suggestion

My greatest wish is that the readers find

this manuscript helpful in gaining the knowledge they

need for effective learning and practice and in

preparing for examination designed to test that

knowledge Reader comments and suggestions are

welcome and would be very helpful in improving our

manuscript Your contribution may assist many other

students in the coming years

31

REFERENCE

Anonim 2013 Anaerobic Respiration

httpwwwscoolcoukgcsebiologyrespiration

revise-itanaerobic-respiration Accessed

September 10th 2013

Anonim 2013 What is the difference between aerobic

and anaerobic respiration in plants

httpwwwbiologylifeeasyorg4010difference

-between-aerobic-anaerobic-respiration-plants

Accessed September 10th 2013

Campbell Mary K 2006 Biochemistry New York Thomson

Brooks Cole Inc

Dat J Capelli N Folzer H Bourgeade P Badot P-M

(2004) Sensing and signaling during

plantflooding Plant Physiology and

Biochemistry 42 273-282

Dat J Folzer H Parent C Badot P-M Capelli N

(2006) HypoxiastressCurrent Understanding and

Perspectives In Teixeira da Silva JA (Ed)

Floriculture Ornamental and Plant

Biotechnology Advances and Topical Issues (Vol

3) Global Science Books Isleworth United

Kingdompp 664-674

C Electron

Transport System

We noted earlier that one of the principal

functions of the respiration is to retrieve in

useful form some of the energy initially stored in

assimilates Our traditional measure of useful

energy in most processes is the number of ATP

molecules gained or consumed By this measure alone

the yield from both glycolysis and the citric acid

cycle is quite low After two complete turns of the

cycle one molecule of glucose has been completely

oxidized to six molecules of CO2 but only four

molecules of ATP have been produced (a net of two

ATP from glycolysis plus one for each turn of the

cycle) At this point most of the energy associated

with the glucose molecule has been conserved in the

17

form of electron pairs generated by the oxidation of

glycolytic and citric acid cycle intermediates

In this section we will discuss the third stage

of cellular respirationmdashthe transfer of electrons

fromNADH and FADH2 to oxygen and the accompanying

conversion of redox energy to ATP The transfer of

electrons fromNADHand FADH2 to oxygen involves a

sequence of electron carriers arranged in an

electron transport chainMembrane fractionation

studies have shown that the enzymes and electron

carriers making up the electron transport chain are

organized predominantly into four large

multimolecular complexes (complexes ImdashIV) and two

mobile carriers located in the inner mitochondrial

membrane (Figure 109) In this sense there are a

great number of similarities between the

mitochondrial inner membrane and the thylakoid

membranes of the chloroplast (compare Figure 109

with Figure 76) This is not unexpected since the

principal function of each membrane is energy

transformation and many of the same or similar

components are involved

The Components of the Electron Transport Chain

The electron transport chain of the

mitochondria is the means by which electrons are

18

removed from the reduced carrier NADH and

transferred to oxygen to yield H2O

1) NADH

NADH is generated in the matrix by the reactions

of pyruvate dehydrogenase isocitrate

dehydrogenase α-ketoglutarate dehydrogenase and

malate dehyrogenase The electron transport chain

begins with reoxidizing NADH to form NAD+ and

channeling the electrons into the formation of

reduced coenzymes Important to note that NADH

transfers 2 electrons at a time in the form of a

hydride

NAD+ + 2e- + H+ 1048581 NADH Eorsquo = minus0315 V

2) Flavoproteins

Flavoproteins have either a FAD (flavin adenosine

dinucleotide) or a FMN (flavin mononucleotide)

prosthetic group Flavoproteins can accept or

donate electrons one at time or two at a time

Thus they are often intermediaries between two

electron acceptorsdonors and one electron

acceptorsdonors For flavoproteins the typical

standard reduction potentials are around 0 V

FAD + 2e- + 2H+ 1048581 FADH2 FMN + 2e- + 2H+ 1048581 FMNH2

Eorsquo asymp 0 V

19

3) Coenzyme Q (CoQ)

aka ubiquinone (UQ) Shown below CoQ has ten

repeating isoprene units which make it

insoluble in water but soluble in the

hydrophobic lipid bylayer Coenzyme Q is a

versatile cofactor because it is a soluble

electron carrier in the hydrophobic bilipid layer

of the

inner mitochondrial membrane Like flavoproteins

CoQ can acceptdonate electrons one at a time or

two at a time

Q + 2e- + 2H+ 1048581 QH2 Eorsquo =0060 V

Q + e- + H+ 1048581 1048581QH Eorsquo =0030 V

1048581QH + e- + H+ 1048581 QH2 Eorsquo =0190 V

4) Cytochromes

Cytochromes are proteins that contain heme

prosthetic groups which function as

one electron carriers The heme iron is involved

in one electron transfers involving the Fe2+ and

Fe3+ oxidation states Cytochromes are named by

their absorption spectra which depends

on the porphyrin structure and environment The

example shown is the heme prosthetic group of

cytochrome b Cytochrome b contains the same iron

porphyrin found in hemoglobin and myoglobin

20

Other cytochromes we will encounter in the

electron transport complexes are cytochromes b

c c1 a and a3

5) Iron-Sulfur Proteins

In the electron transport chain we will encounter

many iron-sulfer proteins which participate in

one electron transfers involving the the Fe2+ and

Fe3+ oxidation states These are non-heme iron-

sulfur proteinsThe simplest iron-sulfer protein

is FeS in which iron is tetrahedrally coordinated

by four cysteines The second form is Fe2S2 which

contains two irons complexed to 2 cysteine

residues and two inorganic sulfides The third

form is Fe3S4 which contains 3 iron atoms

coordinated to three cysteine residues and 4

inorganic sulfides The last form is the most

complicated Fe4S4 which contains 4 iron atoms

coordinated to 4 cysteine residues and 4

inorganic sulfides

6) Copper Proteins

Copper bound proteins participate in one electron

transfers involving the Cu+ and Cu2+ oxidation

states

21

Overview of the Electron Transport Chain

Electrons move along the electron transport

chain going from donor to acceptor until they reach

oxygen the ultimate electron acceptor The standard

reduction potentials of the electron carriers are

between the NADHNAD+ couple (-0315 V) and the

oxygenH2O couple (0816 V) as on the next page The

components of the electron transport chain are

organized into 4 complexes Each complex contains

several different electron carriers

1 Complex I also known as the NADH-coenzyme Q

reductase or NADH dehydrogenase

2 Complex II also known as succinate-coenzyme Q

reductase or succinate dehydrogenase

3 Complex III also known as coenzyme Q reductase

4 Complex IV also known as cytochrome c reductase

The free energy necessary to generate ATP

is extracted from the oxidation of NADH and

FADH2 by the electron transport chain It

consists of four protein complexes and single

protein chain of cytochrome c through which

electrons pass from lower to higher standard

redox potentials Electrons are carried from complex

I and II to complex III by the membrane (lipid)

soluble coenzyme Q (CoQ or ubiquinone) and between

22

complexes III and IV by the peripheral membrane

protein cytochrome c

In the following I give the overall

reactions catalysed by corresponding protein

complexes in the mitochondrial electron transport

chain

1 Complex I

Complex I is also called NADH-Coenzyme Q

reductase because this large protein complex

transfers 2 electrons from NADH to coenzyme Q

Complex I was formerly known as NADH dehydrogenase

Complex I is huge 850000 kD and is composed of more

than thirty subunits It contains a FMN prosthetic

group and seven or more Fe-S clusters This complex

has between 20-26 iron atoms boundThe prosthetic

group FMN is absolutely required for activity

Therefore this complex is a flavoprotein This

complex binds NADH transfers two electrons in the

form of a hydride to FMN to produce NAD+ and FMNH2

23

The subsequent steps involve the transfer of

electrons one at a time to a series of iron-sulfer

complexes that includes both 2Fe-2S and 4Fe-4S

clusters Note the importance of FMN First it

functions as a 2 electron acceptor in the hydride

transfer from NADH Second it functions as a 1

electron donor to the series of iron sulfur clusters

FMN and FAD often play crucial links between 2

electron transfer agents and 1 electron transfer

agents The final step of this complex is the

transfer of 2 electrons one at a time to coenzyme Q

CoQ like FMN and FAD can function as a 2 electron

donoracceptor and as a 1 electron donoracceptor

CoQ is a mobile electron carrier because its

isoprenoid tail makes it highly hydrophobic and

lipophillic It diffuses freely in the bilipid layer

of the inner mitochondrial membrane

The process of transferring electrons from NADH

to CoQ by complex I results in the net transport of

protons from the matrix side of the inner

mitochondrial membrane to the inter membrane space

where the H+ ions accumulate generating a proton

motive force The intermembrane space side of the

inner membrane is referred to as the P face (P

standing for positive) The matrix side of the inner

membrane is referred to as the the N face The

24

transport of electrons from NADH to CoQ is coupled to

the transport of protons across the membrane This is

an example of active transport The stiochiometry is

4 H+ transported per 2 electrons

2 Complex II

Believe it or not you are already familiar

with Complex II It is none other than succinate

dehydrogenase The only enzyme of the citric acid

cycle that is an integral membrane protein This

complex is composed of four subunits 2 of which are

iron-sulfur proteins and the other two subunits

together bind FAD through a covalent link to a

histidine residue These two subunits are called

flavoprotein 2 or FP2 Complex II contains 3 Fe-S

centers 1 4Fe-4S cluster 1 3Fe-4S cluster and 1

2Fe- 2S cluster In the first step of this complex

succinate is bound and a hydride is transferred to

FAD to generate FADH2 and fumarate FADH2 then

transfers its electrons one at a time to the Fe-S

centers Thus once again FAD functions as 2 electron

acceptor and a 1 electron donor The final step of

this complex is the transfer of 2 electrons one at a

time to coenzyme Q to produce CoQH2 For complex II

the standard free energy change of the overall

reaction is too small to drive the transport of

protons across the inner mitochondrial membrane This

25

accounts for the 15 ATPrsquos generated per FADH2

compared with the 25 ATPrsquos generated per NADH

3 Complex III

This complex is also known as coenzyme Q-

cytochrome c reductase because it passes the

electrons form CoQH2 to cyt c through a very unique

electron transport pathway called the Q-cycle Shown

to the left are the porphyrins found in cytochromes

Cytochrome b contains the same iron protoporphyrin as

hemoglobin and myoglobin The c cytochromes contain

heme c through covalent attachment by cysteine

residues Cytochrome a is found in two forms in

complex IV In complex III we find two b-type

cytochromes and one c-type cytochrome Complex III is

complex and we have a crystal structure

4 Complex IV

Complex IV is also known as cytochrome c oxidase

because it accepts the electrons from cytochrome c

and directs them towards the four electron reduction

of O2 to form 2 molecules of H2O

4 cyt c (Fe2+) + 4 H+ + O2 4 cyt c (Fe3+) +

2H2O

26

Cytochrome c oxidase contains 2 heme

centerscytochrome a and cytochrome a3 and two copper

proteins Each of the protein bound coppers are

associated with one of the cytochromes The copper

sites are called CuA and CuB CuA is associated with

cytochrome a and is shown to the left CuB is

associated with cytochrome a3 The copper sites

function as 1 electron carriers cycling between the

cuprous state Cu+ and the cupric state Cu2+ Just

like iron containing proteins they transfer

electrons one at a time Cytochrome c is bound on the

P-face of the membrane and transfers its electron to

CuA The oxidized cytochrome c dissociates CuA then

transfers the electron to cytochrome a The protein

bound CuA and the iron bound in cytochrome a are 15 Aring

apart In contrast the CuB and the iron bound in

cytochrome a3 are very close to each other forming a

binuclear metal center shown below

27

Electron transport chains are associated with

membranes such as the mitochondrial membrane in

eukaryotic organisms the thylakoid membrane of

photosynthetic organisms and the plasma membrane of

prokaryotic organisms While some of the details

differ electron transport chains in all organisms

operate on the same basic principles

Excited electrons are brought to the electron

transport chain by electron carriers such as NADH

and FADH2 In photosynthetic organisms the electrons

are donated by water and excited by light energy

The electron donor is oxidized as the electrons are

passed to the first protein complex in the chain As

it accepts the electrons the protein complex is

reduced Next the electrons are passed from the

28

first protein complex which becomes oxidized to

the second protein complex which becomes reduced

Each of these redox reactions is slightly exergonic

or energy releasing Some of this energy is used to

move protons across the membrane against their

concentration gradient With each successive

transfer of electrons the original excited

electrons lose some of their energy After several

successive redox reactions the electrons are

donated to a terminal electron acceptor In aerobic

respiration the terminal electron acceptor is

oxygen In photosynthesis the terminal electron

acceptor is a compound called NADP

The energy in the excited electrons has been

transformed into an electrochemical gradient

represented by the high concentration of protons on

one side of the membrane These protons can diffuse

back across the membrane by passing through a

protein complex called ATP synthase As these

protons pass through ATP synthase down their

concentration gradient they provide the energy

needed to make ATP from ADP and inorganic phosphate

within the cell

29

CHAPTER III

CLOSING

A Conclution

When a hexose is completely oxidized to CO2

and H2O using these three processes Glycolysis

yields two ATP and two NADH per hexose used Each

such NADH oxidized by the electron transport system

30

yields two ATP as dscribed above so glycolysis

contributes a total of six ATP per hexose The Krebs

cycle contributes two ATP per hexose or per two

pyruvates when succinyl CoA is cleaved to succinate

and CoASH This cycle also produces eight NADH per

hexose within the mitochondrial matrix by oxidative

phosporylation each of these NADH yields three ATP

or 24 per hexose Each FADH2 from the Krebs cycle

yields two ATP by oxydative phosporylation or four

per hexose (two pyruvates) The total contribution of

the Krebs cycle is then 30 Adding these 30 to the 6

from glycolysis leads to a total of 36 per hexose

completely respired by these process

B Suggestion

My greatest wish is that the readers find

this manuscript helpful in gaining the knowledge they

need for effective learning and practice and in

preparing for examination designed to test that

knowledge Reader comments and suggestions are

welcome and would be very helpful in improving our

manuscript Your contribution may assist many other

students in the coming years

31

REFERENCE

Anonim 2013 Anaerobic Respiration

httpwwwscoolcoukgcsebiologyrespiration

revise-itanaerobic-respiration Accessed

September 10th 2013

Anonim 2013 What is the difference between aerobic

and anaerobic respiration in plants

httpwwwbiologylifeeasyorg4010difference

-between-aerobic-anaerobic-respiration-plants

Accessed September 10th 2013

Campbell Mary K 2006 Biochemistry New York Thomson

Brooks Cole Inc

Dat J Capelli N Folzer H Bourgeade P Badot P-M

(2004) Sensing and signaling during

plantflooding Plant Physiology and

Biochemistry 42 273-282

Dat J Folzer H Parent C Badot P-M Capelli N

(2006) HypoxiastressCurrent Understanding and

Perspectives In Teixeira da Silva JA (Ed)

Floriculture Ornamental and Plant

Biotechnology Advances and Topical Issues (Vol

3) Global Science Books Isleworth United

Kingdompp 664-674

form of electron pairs generated by the oxidation of

glycolytic and citric acid cycle intermediates

In this section we will discuss the third stage

of cellular respirationmdashthe transfer of electrons

fromNADH and FADH2 to oxygen and the accompanying

conversion of redox energy to ATP The transfer of

electrons fromNADHand FADH2 to oxygen involves a

sequence of electron carriers arranged in an

electron transport chainMembrane fractionation

studies have shown that the enzymes and electron

carriers making up the electron transport chain are

organized predominantly into four large

multimolecular complexes (complexes ImdashIV) and two

mobile carriers located in the inner mitochondrial

membrane (Figure 109) In this sense there are a

great number of similarities between the

mitochondrial inner membrane and the thylakoid

membranes of the chloroplast (compare Figure 109

with Figure 76) This is not unexpected since the

principal function of each membrane is energy

transformation and many of the same or similar

components are involved

The Components of the Electron Transport Chain

The electron transport chain of the

mitochondria is the means by which electrons are

18

removed from the reduced carrier NADH and

transferred to oxygen to yield H2O

1) NADH

NADH is generated in the matrix by the reactions

of pyruvate dehydrogenase isocitrate

dehydrogenase α-ketoglutarate dehydrogenase and

malate dehyrogenase The electron transport chain

begins with reoxidizing NADH to form NAD+ and

channeling the electrons into the formation of

reduced coenzymes Important to note that NADH

transfers 2 electrons at a time in the form of a

hydride

NAD+ + 2e- + H+ 1048581 NADH Eorsquo = minus0315 V

2) Flavoproteins

Flavoproteins have either a FAD (flavin adenosine

dinucleotide) or a FMN (flavin mononucleotide)

prosthetic group Flavoproteins can accept or

donate electrons one at time or two at a time

Thus they are often intermediaries between two

electron acceptorsdonors and one electron

acceptorsdonors For flavoproteins the typical

standard reduction potentials are around 0 V

FAD + 2e- + 2H+ 1048581 FADH2 FMN + 2e- + 2H+ 1048581 FMNH2

Eorsquo asymp 0 V

19

3) Coenzyme Q (CoQ)

aka ubiquinone (UQ) Shown below CoQ has ten

repeating isoprene units which make it

insoluble in water but soluble in the

hydrophobic lipid bylayer Coenzyme Q is a

versatile cofactor because it is a soluble

electron carrier in the hydrophobic bilipid layer

of the

inner mitochondrial membrane Like flavoproteins

CoQ can acceptdonate electrons one at a time or

two at a time

Q + 2e- + 2H+ 1048581 QH2 Eorsquo =0060 V

Q + e- + H+ 1048581 1048581QH Eorsquo =0030 V

1048581QH + e- + H+ 1048581 QH2 Eorsquo =0190 V

4) Cytochromes

Cytochromes are proteins that contain heme

prosthetic groups which function as

one electron carriers The heme iron is involved

in one electron transfers involving the Fe2+ and

Fe3+ oxidation states Cytochromes are named by

their absorption spectra which depends

on the porphyrin structure and environment The

example shown is the heme prosthetic group of

cytochrome b Cytochrome b contains the same iron

porphyrin found in hemoglobin and myoglobin

20

Other cytochromes we will encounter in the

electron transport complexes are cytochromes b

c c1 a and a3

5) Iron-Sulfur Proteins

In the electron transport chain we will encounter

many iron-sulfer proteins which participate in

one electron transfers involving the the Fe2+ and

Fe3+ oxidation states These are non-heme iron-

sulfur proteinsThe simplest iron-sulfer protein

is FeS in which iron is tetrahedrally coordinated

by four cysteines The second form is Fe2S2 which

contains two irons complexed to 2 cysteine

residues and two inorganic sulfides The third

form is Fe3S4 which contains 3 iron atoms

coordinated to three cysteine residues and 4

inorganic sulfides The last form is the most

complicated Fe4S4 which contains 4 iron atoms

coordinated to 4 cysteine residues and 4

inorganic sulfides

6) Copper Proteins

Copper bound proteins participate in one electron

transfers involving the Cu+ and Cu2+ oxidation

states

21

Overview of the Electron Transport Chain

Electrons move along the electron transport

chain going from donor to acceptor until they reach

oxygen the ultimate electron acceptor The standard

reduction potentials of the electron carriers are

between the NADHNAD+ couple (-0315 V) and the

oxygenH2O couple (0816 V) as on the next page The

components of the electron transport chain are

organized into 4 complexes Each complex contains

several different electron carriers

1 Complex I also known as the NADH-coenzyme Q

reductase or NADH dehydrogenase

2 Complex II also known as succinate-coenzyme Q

reductase or succinate dehydrogenase

3 Complex III also known as coenzyme Q reductase

4 Complex IV also known as cytochrome c reductase

The free energy necessary to generate ATP

is extracted from the oxidation of NADH and

FADH2 by the electron transport chain It

consists of four protein complexes and single

protein chain of cytochrome c through which

electrons pass from lower to higher standard

redox potentials Electrons are carried from complex

I and II to complex III by the membrane (lipid)

soluble coenzyme Q (CoQ or ubiquinone) and between

22

complexes III and IV by the peripheral membrane

protein cytochrome c

In the following I give the overall

reactions catalysed by corresponding protein

complexes in the mitochondrial electron transport

chain

1 Complex I

Complex I is also called NADH-Coenzyme Q

reductase because this large protein complex

transfers 2 electrons from NADH to coenzyme Q

Complex I was formerly known as NADH dehydrogenase

Complex I is huge 850000 kD and is composed of more

than thirty subunits It contains a FMN prosthetic

group and seven or more Fe-S clusters This complex

has between 20-26 iron atoms boundThe prosthetic

group FMN is absolutely required for activity

Therefore this complex is a flavoprotein This

complex binds NADH transfers two electrons in the

form of a hydride to FMN to produce NAD+ and FMNH2

23

The subsequent steps involve the transfer of

electrons one at a time to a series of iron-sulfer

complexes that includes both 2Fe-2S and 4Fe-4S

clusters Note the importance of FMN First it

functions as a 2 electron acceptor in the hydride

transfer from NADH Second it functions as a 1

electron donor to the series of iron sulfur clusters

FMN and FAD often play crucial links between 2

electron transfer agents and 1 electron transfer

agents The final step of this complex is the

transfer of 2 electrons one at a time to coenzyme Q

CoQ like FMN and FAD can function as a 2 electron

donoracceptor and as a 1 electron donoracceptor

CoQ is a mobile electron carrier because its

isoprenoid tail makes it highly hydrophobic and

lipophillic It diffuses freely in the bilipid layer

of the inner mitochondrial membrane

The process of transferring electrons from NADH

to CoQ by complex I results in the net transport of

protons from the matrix side of the inner

mitochondrial membrane to the inter membrane space

where the H+ ions accumulate generating a proton

motive force The intermembrane space side of the

inner membrane is referred to as the P face (P

standing for positive) The matrix side of the inner

membrane is referred to as the the N face The

24

transport of electrons from NADH to CoQ is coupled to

the transport of protons across the membrane This is

an example of active transport The stiochiometry is

4 H+ transported per 2 electrons

2 Complex II

Believe it or not you are already familiar

with Complex II It is none other than succinate

dehydrogenase The only enzyme of the citric acid

cycle that is an integral membrane protein This

complex is composed of four subunits 2 of which are

iron-sulfur proteins and the other two subunits

together bind FAD through a covalent link to a

histidine residue These two subunits are called

flavoprotein 2 or FP2 Complex II contains 3 Fe-S

centers 1 4Fe-4S cluster 1 3Fe-4S cluster and 1

2Fe- 2S cluster In the first step of this complex

succinate is bound and a hydride is transferred to

FAD to generate FADH2 and fumarate FADH2 then

transfers its electrons one at a time to the Fe-S

centers Thus once again FAD functions as 2 electron

acceptor and a 1 electron donor The final step of

this complex is the transfer of 2 electrons one at a

time to coenzyme Q to produce CoQH2 For complex II

the standard free energy change of the overall

reaction is too small to drive the transport of

protons across the inner mitochondrial membrane This

25

accounts for the 15 ATPrsquos generated per FADH2

compared with the 25 ATPrsquos generated per NADH

3 Complex III

This complex is also known as coenzyme Q-

cytochrome c reductase because it passes the

electrons form CoQH2 to cyt c through a very unique

electron transport pathway called the Q-cycle Shown

to the left are the porphyrins found in cytochromes

Cytochrome b contains the same iron protoporphyrin as

hemoglobin and myoglobin The c cytochromes contain

heme c through covalent attachment by cysteine

residues Cytochrome a is found in two forms in

complex IV In complex III we find two b-type

cytochromes and one c-type cytochrome Complex III is

complex and we have a crystal structure

4 Complex IV

Complex IV is also known as cytochrome c oxidase

because it accepts the electrons from cytochrome c

and directs them towards the four electron reduction

of O2 to form 2 molecules of H2O

4 cyt c (Fe2+) + 4 H+ + O2 4 cyt c (Fe3+) +

2H2O

26

Cytochrome c oxidase contains 2 heme

centerscytochrome a and cytochrome a3 and two copper

proteins Each of the protein bound coppers are

associated with one of the cytochromes The copper

sites are called CuA and CuB CuA is associated with

cytochrome a and is shown to the left CuB is

associated with cytochrome a3 The copper sites

function as 1 electron carriers cycling between the

cuprous state Cu+ and the cupric state Cu2+ Just

like iron containing proteins they transfer

electrons one at a time Cytochrome c is bound on the

P-face of the membrane and transfers its electron to

CuA The oxidized cytochrome c dissociates CuA then

transfers the electron to cytochrome a The protein

bound CuA and the iron bound in cytochrome a are 15 Aring

apart In contrast the CuB and the iron bound in

cytochrome a3 are very close to each other forming a

binuclear metal center shown below

27

Electron transport chains are associated with

membranes such as the mitochondrial membrane in

eukaryotic organisms the thylakoid membrane of

photosynthetic organisms and the plasma membrane of

prokaryotic organisms While some of the details

differ electron transport chains in all organisms

operate on the same basic principles

Excited electrons are brought to the electron

transport chain by electron carriers such as NADH

and FADH2 In photosynthetic organisms the electrons

are donated by water and excited by light energy

The electron donor is oxidized as the electrons are

passed to the first protein complex in the chain As

it accepts the electrons the protein complex is

reduced Next the electrons are passed from the

28

first protein complex which becomes oxidized to

the second protein complex which becomes reduced

Each of these redox reactions is slightly exergonic

or energy releasing Some of this energy is used to

move protons across the membrane against their

concentration gradient With each successive

transfer of electrons the original excited

electrons lose some of their energy After several

successive redox reactions the electrons are

donated to a terminal electron acceptor In aerobic

respiration the terminal electron acceptor is

oxygen In photosynthesis the terminal electron

acceptor is a compound called NADP

The energy in the excited electrons has been

transformed into an electrochemical gradient

represented by the high concentration of protons on

one side of the membrane These protons can diffuse

back across the membrane by passing through a

protein complex called ATP synthase As these

protons pass through ATP synthase down their

concentration gradient they provide the energy

needed to make ATP from ADP and inorganic phosphate

within the cell

29

CHAPTER III

CLOSING

A Conclution

When a hexose is completely oxidized to CO2

and H2O using these three processes Glycolysis

yields two ATP and two NADH per hexose used Each

such NADH oxidized by the electron transport system

30

yields two ATP as dscribed above so glycolysis

contributes a total of six ATP per hexose The Krebs

cycle contributes two ATP per hexose or per two

pyruvates when succinyl CoA is cleaved to succinate

and CoASH This cycle also produces eight NADH per

hexose within the mitochondrial matrix by oxidative

phosporylation each of these NADH yields three ATP

or 24 per hexose Each FADH2 from the Krebs cycle

yields two ATP by oxydative phosporylation or four

per hexose (two pyruvates) The total contribution of

the Krebs cycle is then 30 Adding these 30 to the 6

from glycolysis leads to a total of 36 per hexose

completely respired by these process

B Suggestion

My greatest wish is that the readers find

this manuscript helpful in gaining the knowledge they

need for effective learning and practice and in

preparing for examination designed to test that

knowledge Reader comments and suggestions are

welcome and would be very helpful in improving our

manuscript Your contribution may assist many other

students in the coming years

31

REFERENCE

Anonim 2013 Anaerobic Respiration

httpwwwscoolcoukgcsebiologyrespiration

revise-itanaerobic-respiration Accessed

September 10th 2013

Anonim 2013 What is the difference between aerobic

and anaerobic respiration in plants

httpwwwbiologylifeeasyorg4010difference

-between-aerobic-anaerobic-respiration-plants

Accessed September 10th 2013

Campbell Mary K 2006 Biochemistry New York Thomson

Brooks Cole Inc

Dat J Capelli N Folzer H Bourgeade P Badot P-M

(2004) Sensing and signaling during

plantflooding Plant Physiology and

Biochemistry 42 273-282

Dat J Folzer H Parent C Badot P-M Capelli N

(2006) HypoxiastressCurrent Understanding and

Perspectives In Teixeira da Silva JA (Ed)

Floriculture Ornamental and Plant

Biotechnology Advances and Topical Issues (Vol

3) Global Science Books Isleworth United

Kingdompp 664-674

removed from the reduced carrier NADH and

transferred to oxygen to yield H2O

1) NADH

NADH is generated in the matrix by the reactions

of pyruvate dehydrogenase isocitrate

dehydrogenase α-ketoglutarate dehydrogenase and

malate dehyrogenase The electron transport chain

begins with reoxidizing NADH to form NAD+ and

channeling the electrons into the formation of

reduced coenzymes Important to note that NADH

transfers 2 electrons at a time in the form of a

hydride

NAD+ + 2e- + H+ 1048581 NADH Eorsquo = minus0315 V

2) Flavoproteins

Flavoproteins have either a FAD (flavin adenosine

dinucleotide) or a FMN (flavin mononucleotide)

prosthetic group Flavoproteins can accept or

donate electrons one at time or two at a time

Thus they are often intermediaries between two

electron acceptorsdonors and one electron

acceptorsdonors For flavoproteins the typical

standard reduction potentials are around 0 V

FAD + 2e- + 2H+ 1048581 FADH2 FMN + 2e- + 2H+ 1048581 FMNH2

Eorsquo asymp 0 V

19

3) Coenzyme Q (CoQ)

aka ubiquinone (UQ) Shown below CoQ has ten

repeating isoprene units which make it

insoluble in water but soluble in the

hydrophobic lipid bylayer Coenzyme Q is a

versatile cofactor because it is a soluble

electron carrier in the hydrophobic bilipid layer

of the

inner mitochondrial membrane Like flavoproteins

CoQ can acceptdonate electrons one at a time or

two at a time

Q + 2e- + 2H+ 1048581 QH2 Eorsquo =0060 V

Q + e- + H+ 1048581 1048581QH Eorsquo =0030 V

1048581QH + e- + H+ 1048581 QH2 Eorsquo =0190 V

4) Cytochromes

Cytochromes are proteins that contain heme

prosthetic groups which function as

one electron carriers The heme iron is involved

in one electron transfers involving the Fe2+ and

Fe3+ oxidation states Cytochromes are named by

their absorption spectra which depends

on the porphyrin structure and environment The

example shown is the heme prosthetic group of

cytochrome b Cytochrome b contains the same iron

porphyrin found in hemoglobin and myoglobin

20

Other cytochromes we will encounter in the

electron transport complexes are cytochromes b

c c1 a and a3

5) Iron-Sulfur Proteins

In the electron transport chain we will encounter

many iron-sulfer proteins which participate in

one electron transfers involving the the Fe2+ and

Fe3+ oxidation states These are non-heme iron-

sulfur proteinsThe simplest iron-sulfer protein

is FeS in which iron is tetrahedrally coordinated

by four cysteines The second form is Fe2S2 which

contains two irons complexed to 2 cysteine

residues and two inorganic sulfides The third

form is Fe3S4 which contains 3 iron atoms

coordinated to three cysteine residues and 4

inorganic sulfides The last form is the most

complicated Fe4S4 which contains 4 iron atoms

coordinated to 4 cysteine residues and 4

inorganic sulfides

6) Copper Proteins

Copper bound proteins participate in one electron

transfers involving the Cu+ and Cu2+ oxidation

states

21

Overview of the Electron Transport Chain

Electrons move along the electron transport

chain going from donor to acceptor until they reach

oxygen the ultimate electron acceptor The standard

reduction potentials of the electron carriers are

between the NADHNAD+ couple (-0315 V) and the

oxygenH2O couple (0816 V) as on the next page The

components of the electron transport chain are

organized into 4 complexes Each complex contains

several different electron carriers

1 Complex I also known as the NADH-coenzyme Q

reductase or NADH dehydrogenase

2 Complex II also known as succinate-coenzyme Q

reductase or succinate dehydrogenase

3 Complex III also known as coenzyme Q reductase

4 Complex IV also known as cytochrome c reductase

The free energy necessary to generate ATP

is extracted from the oxidation of NADH and

FADH2 by the electron transport chain It

consists of four protein complexes and single

protein chain of cytochrome c through which

electrons pass from lower to higher standard

redox potentials Electrons are carried from complex

I and II to complex III by the membrane (lipid)

soluble coenzyme Q (CoQ or ubiquinone) and between

22

complexes III and IV by the peripheral membrane

protein cytochrome c

In the following I give the overall

reactions catalysed by corresponding protein

complexes in the mitochondrial electron transport

chain

1 Complex I

Complex I is also called NADH-Coenzyme Q

reductase because this large protein complex

transfers 2 electrons from NADH to coenzyme Q

Complex I was formerly known as NADH dehydrogenase

Complex I is huge 850000 kD and is composed of more

than thirty subunits It contains a FMN prosthetic

group and seven or more Fe-S clusters This complex

has between 20-26 iron atoms boundThe prosthetic

group FMN is absolutely required for activity

Therefore this complex is a flavoprotein This

complex binds NADH transfers two electrons in the

form of a hydride to FMN to produce NAD+ and FMNH2

23

The subsequent steps involve the transfer of

electrons one at a time to a series of iron-sulfer

complexes that includes both 2Fe-2S and 4Fe-4S

clusters Note the importance of FMN First it

functions as a 2 electron acceptor in the hydride

transfer from NADH Second it functions as a 1

electron donor to the series of iron sulfur clusters

FMN and FAD often play crucial links between 2

electron transfer agents and 1 electron transfer

agents The final step of this complex is the

transfer of 2 electrons one at a time to coenzyme Q

CoQ like FMN and FAD can function as a 2 electron

donoracceptor and as a 1 electron donoracceptor

CoQ is a mobile electron carrier because its

isoprenoid tail makes it highly hydrophobic and

lipophillic It diffuses freely in the bilipid layer

of the inner mitochondrial membrane

The process of transferring electrons from NADH

to CoQ by complex I results in the net transport of

protons from the matrix side of the inner

mitochondrial membrane to the inter membrane space

where the H+ ions accumulate generating a proton

motive force The intermembrane space side of the

inner membrane is referred to as the P face (P

standing for positive) The matrix side of the inner

membrane is referred to as the the N face The

24

transport of electrons from NADH to CoQ is coupled to

the transport of protons across the membrane This is

an example of active transport The stiochiometry is

4 H+ transported per 2 electrons

2 Complex II

Believe it or not you are already familiar

with Complex II It is none other than succinate

dehydrogenase The only enzyme of the citric acid

cycle that is an integral membrane protein This

complex is composed of four subunits 2 of which are

iron-sulfur proteins and the other two subunits

together bind FAD through a covalent link to a

histidine residue These two subunits are called

flavoprotein 2 or FP2 Complex II contains 3 Fe-S

centers 1 4Fe-4S cluster 1 3Fe-4S cluster and 1

2Fe- 2S cluster In the first step of this complex

succinate is bound and a hydride is transferred to

FAD to generate FADH2 and fumarate FADH2 then

transfers its electrons one at a time to the Fe-S

centers Thus once again FAD functions as 2 electron

acceptor and a 1 electron donor The final step of

this complex is the transfer of 2 electrons one at a

time to coenzyme Q to produce CoQH2 For complex II

the standard free energy change of the overall

reaction is too small to drive the transport of

protons across the inner mitochondrial membrane This

25

accounts for the 15 ATPrsquos generated per FADH2

compared with the 25 ATPrsquos generated per NADH

3 Complex III

This complex is also known as coenzyme Q-

cytochrome c reductase because it passes the

electrons form CoQH2 to cyt c through a very unique

electron transport pathway called the Q-cycle Shown

to the left are the porphyrins found in cytochromes

Cytochrome b contains the same iron protoporphyrin as

hemoglobin and myoglobin The c cytochromes contain

heme c through covalent attachment by cysteine

residues Cytochrome a is found in two forms in

complex IV In complex III we find two b-type

cytochromes and one c-type cytochrome Complex III is

complex and we have a crystal structure

4 Complex IV

Complex IV is also known as cytochrome c oxidase

because it accepts the electrons from cytochrome c

and directs them towards the four electron reduction

of O2 to form 2 molecules of H2O

4 cyt c (Fe2+) + 4 H+ + O2 4 cyt c (Fe3+) +

2H2O

26

Cytochrome c oxidase contains 2 heme

centerscytochrome a and cytochrome a3 and two copper

proteins Each of the protein bound coppers are

associated with one of the cytochromes The copper

sites are called CuA and CuB CuA is associated with

cytochrome a and is shown to the left CuB is

associated with cytochrome a3 The copper sites

function as 1 electron carriers cycling between the

cuprous state Cu+ and the cupric state Cu2+ Just

like iron containing proteins they transfer

electrons one at a time Cytochrome c is bound on the

P-face of the membrane and transfers its electron to

CuA The oxidized cytochrome c dissociates CuA then

transfers the electron to cytochrome a The protein

bound CuA and the iron bound in cytochrome a are 15 Aring

apart In contrast the CuB and the iron bound in

cytochrome a3 are very close to each other forming a

binuclear metal center shown below

27

Electron transport chains are associated with

membranes such as the mitochondrial membrane in

eukaryotic organisms the thylakoid membrane of

photosynthetic organisms and the plasma membrane of

prokaryotic organisms While some of the details

differ electron transport chains in all organisms

operate on the same basic principles

Excited electrons are brought to the electron

transport chain by electron carriers such as NADH

and FADH2 In photosynthetic organisms the electrons

are donated by water and excited by light energy

The electron donor is oxidized as the electrons are

passed to the first protein complex in the chain As

it accepts the electrons the protein complex is

reduced Next the electrons are passed from the

28

first protein complex which becomes oxidized to

the second protein complex which becomes reduced

Each of these redox reactions is slightly exergonic

or energy releasing Some of this energy is used to

move protons across the membrane against their

concentration gradient With each successive

transfer of electrons the original excited

electrons lose some of their energy After several

successive redox reactions the electrons are

donated to a terminal electron acceptor In aerobic

respiration the terminal electron acceptor is

oxygen In photosynthesis the terminal electron

acceptor is a compound called NADP

The energy in the excited electrons has been

transformed into an electrochemical gradient

represented by the high concentration of protons on

one side of the membrane These protons can diffuse

back across the membrane by passing through a

protein complex called ATP synthase As these

protons pass through ATP synthase down their

concentration gradient they provide the energy

needed to make ATP from ADP and inorganic phosphate

within the cell

29

CHAPTER III

CLOSING

A Conclution

When a hexose is completely oxidized to CO2

and H2O using these three processes Glycolysis

yields two ATP and two NADH per hexose used Each

such NADH oxidized by the electron transport system

30

yields two ATP as dscribed above so glycolysis

contributes a total of six ATP per hexose The Krebs

cycle contributes two ATP per hexose or per two

pyruvates when succinyl CoA is cleaved to succinate

and CoASH This cycle also produces eight NADH per

hexose within the mitochondrial matrix by oxidative

phosporylation each of these NADH yields three ATP

or 24 per hexose Each FADH2 from the Krebs cycle

yields two ATP by oxydative phosporylation or four

per hexose (two pyruvates) The total contribution of

the Krebs cycle is then 30 Adding these 30 to the 6

from glycolysis leads to a total of 36 per hexose

completely respired by these process

B Suggestion

My greatest wish is that the readers find

this manuscript helpful in gaining the knowledge they

need for effective learning and practice and in

preparing for examination designed to test that

knowledge Reader comments and suggestions are

welcome and would be very helpful in improving our

manuscript Your contribution may assist many other

students in the coming years

31

REFERENCE

Anonim 2013 Anaerobic Respiration

httpwwwscoolcoukgcsebiologyrespiration

revise-itanaerobic-respiration Accessed

September 10th 2013

Anonim 2013 What is the difference between aerobic

and anaerobic respiration in plants

httpwwwbiologylifeeasyorg4010difference

-between-aerobic-anaerobic-respiration-plants

Accessed September 10th 2013

Campbell Mary K 2006 Biochemistry New York Thomson

Brooks Cole Inc

Dat J Capelli N Folzer H Bourgeade P Badot P-M

(2004) Sensing and signaling during

plantflooding Plant Physiology and

Biochemistry 42 273-282

Dat J Folzer H Parent C Badot P-M Capelli N

(2006) HypoxiastressCurrent Understanding and

Perspectives In Teixeira da Silva JA (Ed)

Floriculture Ornamental and Plant

Biotechnology Advances and Topical Issues (Vol

3) Global Science Books Isleworth United

Kingdompp 664-674

3) Coenzyme Q (CoQ)

aka ubiquinone (UQ) Shown below CoQ has ten

repeating isoprene units which make it

insoluble in water but soluble in the

hydrophobic lipid bylayer Coenzyme Q is a

versatile cofactor because it is a soluble

electron carrier in the hydrophobic bilipid layer

of the

inner mitochondrial membrane Like flavoproteins

CoQ can acceptdonate electrons one at a time or

two at a time

Q + 2e- + 2H+ 1048581 QH2 Eorsquo =0060 V

Q + e- + H+ 1048581 1048581QH Eorsquo =0030 V

1048581QH + e- + H+ 1048581 QH2 Eorsquo =0190 V

4) Cytochromes

Cytochromes are proteins that contain heme

prosthetic groups which function as

one electron carriers The heme iron is involved

in one electron transfers involving the Fe2+ and

Fe3+ oxidation states Cytochromes are named by

their absorption spectra which depends

on the porphyrin structure and environment The

example shown is the heme prosthetic group of

cytochrome b Cytochrome b contains the same iron

porphyrin found in hemoglobin and myoglobin

20

Other cytochromes we will encounter in the

electron transport complexes are cytochromes b

c c1 a and a3

5) Iron-Sulfur Proteins

In the electron transport chain we will encounter

many iron-sulfer proteins which participate in

one electron transfers involving the the Fe2+ and

Fe3+ oxidation states These are non-heme iron-

sulfur proteinsThe simplest iron-sulfer protein

is FeS in which iron is tetrahedrally coordinated

by four cysteines The second form is Fe2S2 which

contains two irons complexed to 2 cysteine

residues and two inorganic sulfides The third

form is Fe3S4 which contains 3 iron atoms

coordinated to three cysteine residues and 4

inorganic sulfides The last form is the most

complicated Fe4S4 which contains 4 iron atoms

coordinated to 4 cysteine residues and 4

inorganic sulfides

6) Copper Proteins

Copper bound proteins participate in one electron

transfers involving the Cu+ and Cu2+ oxidation

states

21

Overview of the Electron Transport Chain

Electrons move along the electron transport

chain going from donor to acceptor until they reach

oxygen the ultimate electron acceptor The standard

reduction potentials of the electron carriers are

between the NADHNAD+ couple (-0315 V) and the

oxygenH2O couple (0816 V) as on the next page The

components of the electron transport chain are

organized into 4 complexes Each complex contains

several different electron carriers

1 Complex I also known as the NADH-coenzyme Q

reductase or NADH dehydrogenase

2 Complex II also known as succinate-coenzyme Q

reductase or succinate dehydrogenase

3 Complex III also known as coenzyme Q reductase

4 Complex IV also known as cytochrome c reductase

The free energy necessary to generate ATP

is extracted from the oxidation of NADH and

FADH2 by the electron transport chain It

consists of four protein complexes and single

protein chain of cytochrome c through which

electrons pass from lower to higher standard

redox potentials Electrons are carried from complex

I and II to complex III by the membrane (lipid)

soluble coenzyme Q (CoQ or ubiquinone) and between

22

complexes III and IV by the peripheral membrane

protein cytochrome c

In the following I give the overall

reactions catalysed by corresponding protein

complexes in the mitochondrial electron transport

chain

1 Complex I

Complex I is also called NADH-Coenzyme Q

reductase because this large protein complex

transfers 2 electrons from NADH to coenzyme Q

Complex I was formerly known as NADH dehydrogenase

Complex I is huge 850000 kD and is composed of more

than thirty subunits It contains a FMN prosthetic

group and seven or more Fe-S clusters This complex

has between 20-26 iron atoms boundThe prosthetic

group FMN is absolutely required for activity

Therefore this complex is a flavoprotein This

complex binds NADH transfers two electrons in the

form of a hydride to FMN to produce NAD+ and FMNH2

23

The subsequent steps involve the transfer of

electrons one at a time to a series of iron-sulfer

complexes that includes both 2Fe-2S and 4Fe-4S

clusters Note the importance of FMN First it

functions as a 2 electron acceptor in the hydride

transfer from NADH Second it functions as a 1

electron donor to the series of iron sulfur clusters

FMN and FAD often play crucial links between 2

electron transfer agents and 1 electron transfer

agents The final step of this complex is the

transfer of 2 electrons one at a time to coenzyme Q

CoQ like FMN and FAD can function as a 2 electron

donoracceptor and as a 1 electron donoracceptor

CoQ is a mobile electron carrier because its

isoprenoid tail makes it highly hydrophobic and

lipophillic It diffuses freely in the bilipid layer

of the inner mitochondrial membrane

The process of transferring electrons from NADH

to CoQ by complex I results in the net transport of

protons from the matrix side of the inner

mitochondrial membrane to the inter membrane space

where the H+ ions accumulate generating a proton

motive force The intermembrane space side of the

inner membrane is referred to as the P face (P

standing for positive) The matrix side of the inner

membrane is referred to as the the N face The

24

transport of electrons from NADH to CoQ is coupled to

the transport of protons across the membrane This is

an example of active transport The stiochiometry is

4 H+ transported per 2 electrons

2 Complex II

Believe it or not you are already familiar

with Complex II It is none other than succinate

dehydrogenase The only enzyme of the citric acid

cycle that is an integral membrane protein This

complex is composed of four subunits 2 of which are

iron-sulfur proteins and the other two subunits

together bind FAD through a covalent link to a

histidine residue These two subunits are called

flavoprotein 2 or FP2 Complex II contains 3 Fe-S

centers 1 4Fe-4S cluster 1 3Fe-4S cluster and 1

2Fe- 2S cluster In the first step of this complex

succinate is bound and a hydride is transferred to

FAD to generate FADH2 and fumarate FADH2 then

transfers its electrons one at a time to the Fe-S

centers Thus once again FAD functions as 2 electron

acceptor and a 1 electron donor The final step of

this complex is the transfer of 2 electrons one at a

time to coenzyme Q to produce CoQH2 For complex II

the standard free energy change of the overall

reaction is too small to drive the transport of

protons across the inner mitochondrial membrane This

25

accounts for the 15 ATPrsquos generated per FADH2

compared with the 25 ATPrsquos generated per NADH

3 Complex III

This complex is also known as coenzyme Q-

cytochrome c reductase because it passes the

electrons form CoQH2 to cyt c through a very unique

electron transport pathway called the Q-cycle Shown

to the left are the porphyrins found in cytochromes

Cytochrome b contains the same iron protoporphyrin as

hemoglobin and myoglobin The c cytochromes contain

heme c through covalent attachment by cysteine

residues Cytochrome a is found in two forms in

complex IV In complex III we find two b-type

cytochromes and one c-type cytochrome Complex III is

complex and we have a crystal structure

4 Complex IV

Complex IV is also known as cytochrome c oxidase

because it accepts the electrons from cytochrome c

and directs them towards the four electron reduction

of O2 to form 2 molecules of H2O

4 cyt c (Fe2+) + 4 H+ + O2 4 cyt c (Fe3+) +

2H2O

26

Cytochrome c oxidase contains 2 heme

centerscytochrome a and cytochrome a3 and two copper

proteins Each of the protein bound coppers are

associated with one of the cytochromes The copper

sites are called CuA and CuB CuA is associated with

cytochrome a and is shown to the left CuB is

associated with cytochrome a3 The copper sites

function as 1 electron carriers cycling between the

cuprous state Cu+ and the cupric state Cu2+ Just

like iron containing proteins they transfer

electrons one at a time Cytochrome c is bound on the

P-face of the membrane and transfers its electron to

CuA The oxidized cytochrome c dissociates CuA then

transfers the electron to cytochrome a The protein

bound CuA and the iron bound in cytochrome a are 15 Aring

apart In contrast the CuB and the iron bound in

cytochrome a3 are very close to each other forming a

binuclear metal center shown below

27

Electron transport chains are associated with

membranes such as the mitochondrial membrane in

eukaryotic organisms the thylakoid membrane of

photosynthetic organisms and the plasma membrane of

prokaryotic organisms While some of the details

differ electron transport chains in all organisms

operate on the same basic principles

Excited electrons are brought to the electron

transport chain by electron carriers such as NADH

and FADH2 In photosynthetic organisms the electrons

are donated by water and excited by light energy

The electron donor is oxidized as the electrons are

passed to the first protein complex in the chain As

it accepts the electrons the protein complex is

reduced Next the electrons are passed from the

28

first protein complex which becomes oxidized to

the second protein complex which becomes reduced

Each of these redox reactions is slightly exergonic

or energy releasing Some of this energy is used to

move protons across the membrane against their

concentration gradient With each successive

transfer of electrons the original excited

electrons lose some of their energy After several

successive redox reactions the electrons are

donated to a terminal electron acceptor In aerobic

respiration the terminal electron acceptor is

oxygen In photosynthesis the terminal electron

acceptor is a compound called NADP

The energy in the excited electrons has been

transformed into an electrochemical gradient

represented by the high concentration of protons on

one side of the membrane These protons can diffuse

back across the membrane by passing through a

protein complex called ATP synthase As these

protons pass through ATP synthase down their

concentration gradient they provide the energy

needed to make ATP from ADP and inorganic phosphate

within the cell

29

CHAPTER III

CLOSING

A Conclution

When a hexose is completely oxidized to CO2

and H2O using these three processes Glycolysis

yields two ATP and two NADH per hexose used Each

such NADH oxidized by the electron transport system

30

yields two ATP as dscribed above so glycolysis

contributes a total of six ATP per hexose The Krebs

cycle contributes two ATP per hexose or per two

pyruvates when succinyl CoA is cleaved to succinate

and CoASH This cycle also produces eight NADH per

hexose within the mitochondrial matrix by oxidative

phosporylation each of these NADH yields three ATP

or 24 per hexose Each FADH2 from the Krebs cycle

yields two ATP by oxydative phosporylation or four

per hexose (two pyruvates) The total contribution of

the Krebs cycle is then 30 Adding these 30 to the 6

from glycolysis leads to a total of 36 per hexose

completely respired by these process

B Suggestion

My greatest wish is that the readers find

this manuscript helpful in gaining the knowledge they

need for effective learning and practice and in

preparing for examination designed to test that

knowledge Reader comments and suggestions are

welcome and would be very helpful in improving our

manuscript Your contribution may assist many other

students in the coming years

31

REFERENCE

Anonim 2013 Anaerobic Respiration

httpwwwscoolcoukgcsebiologyrespiration

revise-itanaerobic-respiration Accessed

September 10th 2013

Anonim 2013 What is the difference between aerobic

and anaerobic respiration in plants

httpwwwbiologylifeeasyorg4010difference

-between-aerobic-anaerobic-respiration-plants

Accessed September 10th 2013

Campbell Mary K 2006 Biochemistry New York Thomson

Brooks Cole Inc

Dat J Capelli N Folzer H Bourgeade P Badot P-M

(2004) Sensing and signaling during

plantflooding Plant Physiology and

Biochemistry 42 273-282

Dat J Folzer H Parent C Badot P-M Capelli N

(2006) HypoxiastressCurrent Understanding and

Perspectives In Teixeira da Silva JA (Ed)

Floriculture Ornamental and Plant

Biotechnology Advances and Topical Issues (Vol

3) Global Science Books Isleworth United

Kingdompp 664-674

Other cytochromes we will encounter in the

electron transport complexes are cytochromes b

c c1 a and a3

5) Iron-Sulfur Proteins

In the electron transport chain we will encounter

many iron-sulfer proteins which participate in

one electron transfers involving the the Fe2+ and

Fe3+ oxidation states These are non-heme iron-

sulfur proteinsThe simplest iron-sulfer protein

is FeS in which iron is tetrahedrally coordinated

by four cysteines The second form is Fe2S2 which

contains two irons complexed to 2 cysteine

residues and two inorganic sulfides The third

form is Fe3S4 which contains 3 iron atoms

coordinated to three cysteine residues and 4

inorganic sulfides The last form is the most

complicated Fe4S4 which contains 4 iron atoms

coordinated to 4 cysteine residues and 4

inorganic sulfides

6) Copper Proteins

Copper bound proteins participate in one electron

transfers involving the Cu+ and Cu2+ oxidation

states

21

Overview of the Electron Transport Chain

Electrons move along the electron transport

chain going from donor to acceptor until they reach

oxygen the ultimate electron acceptor The standard

reduction potentials of the electron carriers are

between the NADHNAD+ couple (-0315 V) and the

oxygenH2O couple (0816 V) as on the next page The

components of the electron transport chain are

organized into 4 complexes Each complex contains

several different electron carriers

1 Complex I also known as the NADH-coenzyme Q

reductase or NADH dehydrogenase

2 Complex II also known as succinate-coenzyme Q

reductase or succinate dehydrogenase

3 Complex III also known as coenzyme Q reductase

4 Complex IV also known as cytochrome c reductase

The free energy necessary to generate ATP

is extracted from the oxidation of NADH and

FADH2 by the electron transport chain It

consists of four protein complexes and single

protein chain of cytochrome c through which

electrons pass from lower to higher standard

redox potentials Electrons are carried from complex

I and II to complex III by the membrane (lipid)

soluble coenzyme Q (CoQ or ubiquinone) and between

22

complexes III and IV by the peripheral membrane

protein cytochrome c

In the following I give the overall

reactions catalysed by corresponding protein

complexes in the mitochondrial electron transport

chain

1 Complex I

Complex I is also called NADH-Coenzyme Q

reductase because this large protein complex

transfers 2 electrons from NADH to coenzyme Q

Complex I was formerly known as NADH dehydrogenase

Complex I is huge 850000 kD and is composed of more

than thirty subunits It contains a FMN prosthetic

group and seven or more Fe-S clusters This complex

has between 20-26 iron atoms boundThe prosthetic

group FMN is absolutely required for activity

Therefore this complex is a flavoprotein This

complex binds NADH transfers two electrons in the

form of a hydride to FMN to produce NAD+ and FMNH2

23

The subsequent steps involve the transfer of

electrons one at a time to a series of iron-sulfer

complexes that includes both 2Fe-2S and 4Fe-4S

clusters Note the importance of FMN First it

functions as a 2 electron acceptor in the hydride

transfer from NADH Second it functions as a 1

electron donor to the series of iron sulfur clusters

FMN and FAD often play crucial links between 2

electron transfer agents and 1 electron transfer

agents The final step of this complex is the

transfer of 2 electrons one at a time to coenzyme Q

CoQ like FMN and FAD can function as a 2 electron

donoracceptor and as a 1 electron donoracceptor

CoQ is a mobile electron carrier because its

isoprenoid tail makes it highly hydrophobic and

lipophillic It diffuses freely in the bilipid layer

of the inner mitochondrial membrane

The process of transferring electrons from NADH

to CoQ by complex I results in the net transport of

protons from the matrix side of the inner

mitochondrial membrane to the inter membrane space

where the H+ ions accumulate generating a proton

motive force The intermembrane space side of the

inner membrane is referred to as the P face (P

standing for positive) The matrix side of the inner

membrane is referred to as the the N face The

24

transport of electrons from NADH to CoQ is coupled to

the transport of protons across the membrane This is

an example of active transport The stiochiometry is

4 H+ transported per 2 electrons

2 Complex II

Believe it or not you are already familiar

with Complex II It is none other than succinate

dehydrogenase The only enzyme of the citric acid

cycle that is an integral membrane protein This

complex is composed of four subunits 2 of which are

iron-sulfur proteins and the other two subunits

together bind FAD through a covalent link to a

histidine residue These two subunits are called

flavoprotein 2 or FP2 Complex II contains 3 Fe-S

centers 1 4Fe-4S cluster 1 3Fe-4S cluster and 1

2Fe- 2S cluster In the first step of this complex

succinate is bound and a hydride is transferred to

FAD to generate FADH2 and fumarate FADH2 then

transfers its electrons one at a time to the Fe-S

centers Thus once again FAD functions as 2 electron

acceptor and a 1 electron donor The final step of

this complex is the transfer of 2 electrons one at a

time to coenzyme Q to produce CoQH2 For complex II

the standard free energy change of the overall

reaction is too small to drive the transport of

protons across the inner mitochondrial membrane This

25

accounts for the 15 ATPrsquos generated per FADH2

compared with the 25 ATPrsquos generated per NADH

3 Complex III

This complex is also known as coenzyme Q-

cytochrome c reductase because it passes the

electrons form CoQH2 to cyt c through a very unique

electron transport pathway called the Q-cycle Shown

to the left are the porphyrins found in cytochromes

Cytochrome b contains the same iron protoporphyrin as

hemoglobin and myoglobin The c cytochromes contain

heme c through covalent attachment by cysteine

residues Cytochrome a is found in two forms in

complex IV In complex III we find two b-type

cytochromes and one c-type cytochrome Complex III is

complex and we have a crystal structure

4 Complex IV

Complex IV is also known as cytochrome c oxidase

because it accepts the electrons from cytochrome c

and directs them towards the four electron reduction

of O2 to form 2 molecules of H2O

4 cyt c (Fe2+) + 4 H+ + O2 4 cyt c (Fe3+) +

2H2O

26

Cytochrome c oxidase contains 2 heme

centerscytochrome a and cytochrome a3 and two copper

proteins Each of the protein bound coppers are

associated with one of the cytochromes The copper

sites are called CuA and CuB CuA is associated with

cytochrome a and is shown to the left CuB is

associated with cytochrome a3 The copper sites

function as 1 electron carriers cycling between the

cuprous state Cu+ and the cupric state Cu2+ Just

like iron containing proteins they transfer

electrons one at a time Cytochrome c is bound on the

P-face of the membrane and transfers its electron to

CuA The oxidized cytochrome c dissociates CuA then

transfers the electron to cytochrome a The protein

bound CuA and the iron bound in cytochrome a are 15 Aring

apart In contrast the CuB and the iron bound in

cytochrome a3 are very close to each other forming a

binuclear metal center shown below

27

Electron transport chains are associated with

membranes such as the mitochondrial membrane in

eukaryotic organisms the thylakoid membrane of

photosynthetic organisms and the plasma membrane of

prokaryotic organisms While some of the details

differ electron transport chains in all organisms

operate on the same basic principles

Excited electrons are brought to the electron

transport chain by electron carriers such as NADH

and FADH2 In photosynthetic organisms the electrons

are donated by water and excited by light energy

The electron donor is oxidized as the electrons are

passed to the first protein complex in the chain As

it accepts the electrons the protein complex is

reduced Next the electrons are passed from the

28

first protein complex which becomes oxidized to

the second protein complex which becomes reduced

Each of these redox reactions is slightly exergonic

or energy releasing Some of this energy is used to

move protons across the membrane against their

concentration gradient With each successive

transfer of electrons the original excited

electrons lose some of their energy After several

successive redox reactions the electrons are

donated to a terminal electron acceptor In aerobic

respiration the terminal electron acceptor is

oxygen In photosynthesis the terminal electron

acceptor is a compound called NADP

The energy in the excited electrons has been

transformed into an electrochemical gradient

represented by the high concentration of protons on

one side of the membrane These protons can diffuse

back across the membrane by passing through a

protein complex called ATP synthase As these

protons pass through ATP synthase down their

concentration gradient they provide the energy

needed to make ATP from ADP and inorganic phosphate

within the cell

29

CHAPTER III

CLOSING

A Conclution

When a hexose is completely oxidized to CO2

and H2O using these three processes Glycolysis

yields two ATP and two NADH per hexose used Each

such NADH oxidized by the electron transport system

30

yields two ATP as dscribed above so glycolysis

contributes a total of six ATP per hexose The Krebs

cycle contributes two ATP per hexose or per two

pyruvates when succinyl CoA is cleaved to succinate

and CoASH This cycle also produces eight NADH per

hexose within the mitochondrial matrix by oxidative

phosporylation each of these NADH yields three ATP

or 24 per hexose Each FADH2 from the Krebs cycle

yields two ATP by oxydative phosporylation or four

per hexose (two pyruvates) The total contribution of

the Krebs cycle is then 30 Adding these 30 to the 6

from glycolysis leads to a total of 36 per hexose

completely respired by these process

B Suggestion

My greatest wish is that the readers find

this manuscript helpful in gaining the knowledge they

need for effective learning and practice and in

preparing for examination designed to test that

knowledge Reader comments and suggestions are

welcome and would be very helpful in improving our

manuscript Your contribution may assist many other

students in the coming years

31

REFERENCE

Anonim 2013 Anaerobic Respiration

httpwwwscoolcoukgcsebiologyrespiration

revise-itanaerobic-respiration Accessed

September 10th 2013

Anonim 2013 What is the difference between aerobic

and anaerobic respiration in plants

httpwwwbiologylifeeasyorg4010difference

-between-aerobic-anaerobic-respiration-plants

Accessed September 10th 2013

Campbell Mary K 2006 Biochemistry New York Thomson

Brooks Cole Inc

Dat J Capelli N Folzer H Bourgeade P Badot P-M

(2004) Sensing and signaling during

plantflooding Plant Physiology and

Biochemistry 42 273-282

Dat J Folzer H Parent C Badot P-M Capelli N

(2006) HypoxiastressCurrent Understanding and

Perspectives In Teixeira da Silva JA (Ed)

Floriculture Ornamental and Plant

Biotechnology Advances and Topical Issues (Vol

3) Global Science Books Isleworth United

Kingdompp 664-674

Overview of the Electron Transport Chain

Electrons move along the electron transport

chain going from donor to acceptor until they reach

oxygen the ultimate electron acceptor The standard

reduction potentials of the electron carriers are

between the NADHNAD+ couple (-0315 V) and the

oxygenH2O couple (0816 V) as on the next page The

components of the electron transport chain are

organized into 4 complexes Each complex contains

several different electron carriers

1 Complex I also known as the NADH-coenzyme Q

reductase or NADH dehydrogenase

2 Complex II also known as succinate-coenzyme Q

reductase or succinate dehydrogenase

3 Complex III also known as coenzyme Q reductase

4 Complex IV also known as cytochrome c reductase

The free energy necessary to generate ATP

is extracted from the oxidation of NADH and

FADH2 by the electron transport chain It

consists of four protein complexes and single

protein chain of cytochrome c through which

electrons pass from lower to higher standard

redox potentials Electrons are carried from complex

I and II to complex III by the membrane (lipid)

soluble coenzyme Q (CoQ or ubiquinone) and between

22

complexes III and IV by the peripheral membrane

protein cytochrome c

In the following I give the overall

reactions catalysed by corresponding protein

complexes in the mitochondrial electron transport

chain

1 Complex I

Complex I is also called NADH-Coenzyme Q

reductase because this large protein complex

transfers 2 electrons from NADH to coenzyme Q

Complex I was formerly known as NADH dehydrogenase

Complex I is huge 850000 kD and is composed of more

than thirty subunits It contains a FMN prosthetic

group and seven or more Fe-S clusters This complex

has between 20-26 iron atoms boundThe prosthetic

group FMN is absolutely required for activity

Therefore this complex is a flavoprotein This

complex binds NADH transfers two electrons in the

form of a hydride to FMN to produce NAD+ and FMNH2

23

The subsequent steps involve the transfer of

electrons one at a time to a series of iron-sulfer

complexes that includes both 2Fe-2S and 4Fe-4S

clusters Note the importance of FMN First it

functions as a 2 electron acceptor in the hydride

transfer from NADH Second it functions as a 1

electron donor to the series of iron sulfur clusters

FMN and FAD often play crucial links between 2

electron transfer agents and 1 electron transfer

agents The final step of this complex is the

transfer of 2 electrons one at a time to coenzyme Q

CoQ like FMN and FAD can function as a 2 electron

donoracceptor and as a 1 electron donoracceptor

CoQ is a mobile electron carrier because its

isoprenoid tail makes it highly hydrophobic and

lipophillic It diffuses freely in the bilipid layer

of the inner mitochondrial membrane

The process of transferring electrons from NADH

to CoQ by complex I results in the net transport of

protons from the matrix side of the inner

mitochondrial membrane to the inter membrane space

where the H+ ions accumulate generating a proton

motive force The intermembrane space side of the

inner membrane is referred to as the P face (P

standing for positive) The matrix side of the inner

membrane is referred to as the the N face The

24

transport of electrons from NADH to CoQ is coupled to

the transport of protons across the membrane This is

an example of active transport The stiochiometry is

4 H+ transported per 2 electrons

2 Complex II

Believe it or not you are already familiar

with Complex II It is none other than succinate

dehydrogenase The only enzyme of the citric acid

cycle that is an integral membrane protein This

complex is composed of four subunits 2 of which are

iron-sulfur proteins and the other two subunits

together bind FAD through a covalent link to a

histidine residue These two subunits are called

flavoprotein 2 or FP2 Complex II contains 3 Fe-S

centers 1 4Fe-4S cluster 1 3Fe-4S cluster and 1

2Fe- 2S cluster In the first step of this complex

succinate is bound and a hydride is transferred to

FAD to generate FADH2 and fumarate FADH2 then

transfers its electrons one at a time to the Fe-S

centers Thus once again FAD functions as 2 electron

acceptor and a 1 electron donor The final step of

this complex is the transfer of 2 electrons one at a

time to coenzyme Q to produce CoQH2 For complex II

the standard free energy change of the overall

reaction is too small to drive the transport of

protons across the inner mitochondrial membrane This

25

accounts for the 15 ATPrsquos generated per FADH2

compared with the 25 ATPrsquos generated per NADH

3 Complex III

This complex is also known as coenzyme Q-

cytochrome c reductase because it passes the

electrons form CoQH2 to cyt c through a very unique

electron transport pathway called the Q-cycle Shown

to the left are the porphyrins found in cytochromes

Cytochrome b contains the same iron protoporphyrin as

hemoglobin and myoglobin The c cytochromes contain

heme c through covalent attachment by cysteine

residues Cytochrome a is found in two forms in

complex IV In complex III we find two b-type

cytochromes and one c-type cytochrome Complex III is

complex and we have a crystal structure

4 Complex IV

Complex IV is also known as cytochrome c oxidase

because it accepts the electrons from cytochrome c

and directs them towards the four electron reduction

of O2 to form 2 molecules of H2O

4 cyt c (Fe2+) + 4 H+ + O2 4 cyt c (Fe3+) +

2H2O

26

Cytochrome c oxidase contains 2 heme

centerscytochrome a and cytochrome a3 and two copper

proteins Each of the protein bound coppers are

associated with one of the cytochromes The copper

sites are called CuA and CuB CuA is associated with

cytochrome a and is shown to the left CuB is

associated with cytochrome a3 The copper sites

function as 1 electron carriers cycling between the

cuprous state Cu+ and the cupric state Cu2+ Just

like iron containing proteins they transfer

electrons one at a time Cytochrome c is bound on the

P-face of the membrane and transfers its electron to

CuA The oxidized cytochrome c dissociates CuA then

transfers the electron to cytochrome a The protein

bound CuA and the iron bound in cytochrome a are 15 Aring

apart In contrast the CuB and the iron bound in

cytochrome a3 are very close to each other forming a

binuclear metal center shown below

27

Electron transport chains are associated with

membranes such as the mitochondrial membrane in

eukaryotic organisms the thylakoid membrane of

photosynthetic organisms and the plasma membrane of

prokaryotic organisms While some of the details

differ electron transport chains in all organisms

operate on the same basic principles

Excited electrons are brought to the electron

transport chain by electron carriers such as NADH

and FADH2 In photosynthetic organisms the electrons

are donated by water and excited by light energy

The electron donor is oxidized as the electrons are

passed to the first protein complex in the chain As

it accepts the electrons the protein complex is

reduced Next the electrons are passed from the

28

first protein complex which becomes oxidized to

the second protein complex which becomes reduced

Each of these redox reactions is slightly exergonic

or energy releasing Some of this energy is used to

move protons across the membrane against their

concentration gradient With each successive

transfer of electrons the original excited

electrons lose some of their energy After several

successive redox reactions the electrons are

donated to a terminal electron acceptor In aerobic

respiration the terminal electron acceptor is

oxygen In photosynthesis the terminal electron

acceptor is a compound called NADP

The energy in the excited electrons has been

transformed into an electrochemical gradient

represented by the high concentration of protons on

one side of the membrane These protons can diffuse

back across the membrane by passing through a

protein complex called ATP synthase As these

protons pass through ATP synthase down their

concentration gradient they provide the energy

needed to make ATP from ADP and inorganic phosphate

within the cell

29

CHAPTER III

CLOSING

A Conclution

When a hexose is completely oxidized to CO2

and H2O using these three processes Glycolysis

yields two ATP and two NADH per hexose used Each

such NADH oxidized by the electron transport system

30

yields two ATP as dscribed above so glycolysis

contributes a total of six ATP per hexose The Krebs

cycle contributes two ATP per hexose or per two

pyruvates when succinyl CoA is cleaved to succinate

and CoASH This cycle also produces eight NADH per

hexose within the mitochondrial matrix by oxidative

phosporylation each of these NADH yields three ATP

or 24 per hexose Each FADH2 from the Krebs cycle

yields two ATP by oxydative phosporylation or four

per hexose (two pyruvates) The total contribution of

the Krebs cycle is then 30 Adding these 30 to the 6

from glycolysis leads to a total of 36 per hexose

completely respired by these process

B Suggestion

My greatest wish is that the readers find

this manuscript helpful in gaining the knowledge they

need for effective learning and practice and in

preparing for examination designed to test that

knowledge Reader comments and suggestions are

welcome and would be very helpful in improving our

manuscript Your contribution may assist many other

students in the coming years

31

REFERENCE

Anonim 2013 Anaerobic Respiration

httpwwwscoolcoukgcsebiologyrespiration

revise-itanaerobic-respiration Accessed

September 10th 2013

Anonim 2013 What is the difference between aerobic

and anaerobic respiration in plants

httpwwwbiologylifeeasyorg4010difference

-between-aerobic-anaerobic-respiration-plants

Accessed September 10th 2013

Campbell Mary K 2006 Biochemistry New York Thomson

Brooks Cole Inc

Dat J Capelli N Folzer H Bourgeade P Badot P-M

(2004) Sensing and signaling during

plantflooding Plant Physiology and

Biochemistry 42 273-282

Dat J Folzer H Parent C Badot P-M Capelli N

(2006) HypoxiastressCurrent Understanding and

Perspectives In Teixeira da Silva JA (Ed)

Floriculture Ornamental and Plant

Biotechnology Advances and Topical Issues (Vol

3) Global Science Books Isleworth United

Kingdompp 664-674

complexes III and IV by the peripheral membrane

protein cytochrome c

In the following I give the overall

reactions catalysed by corresponding protein

complexes in the mitochondrial electron transport

chain

1 Complex I

Complex I is also called NADH-Coenzyme Q

reductase because this large protein complex

transfers 2 electrons from NADH to coenzyme Q

Complex I was formerly known as NADH dehydrogenase

Complex I is huge 850000 kD and is composed of more

than thirty subunits It contains a FMN prosthetic

group and seven or more Fe-S clusters This complex

has between 20-26 iron atoms boundThe prosthetic

group FMN is absolutely required for activity

Therefore this complex is a flavoprotein This

complex binds NADH transfers two electrons in the

form of a hydride to FMN to produce NAD+ and FMNH2

23

The subsequent steps involve the transfer of

electrons one at a time to a series of iron-sulfer

complexes that includes both 2Fe-2S and 4Fe-4S

clusters Note the importance of FMN First it

functions as a 2 electron acceptor in the hydride

transfer from NADH Second it functions as a 1

electron donor to the series of iron sulfur clusters

FMN and FAD often play crucial links between 2

electron transfer agents and 1 electron transfer

agents The final step of this complex is the

transfer of 2 electrons one at a time to coenzyme Q

CoQ like FMN and FAD can function as a 2 electron

donoracceptor and as a 1 electron donoracceptor

CoQ is a mobile electron carrier because its

isoprenoid tail makes it highly hydrophobic and

lipophillic It diffuses freely in the bilipid layer

of the inner mitochondrial membrane

The process of transferring electrons from NADH

to CoQ by complex I results in the net transport of

protons from the matrix side of the inner

mitochondrial membrane to the inter membrane space

where the H+ ions accumulate generating a proton

motive force The intermembrane space side of the

inner membrane is referred to as the P face (P

standing for positive) The matrix side of the inner

membrane is referred to as the the N face The

24

transport of electrons from NADH to CoQ is coupled to

the transport of protons across the membrane This is

an example of active transport The stiochiometry is

4 H+ transported per 2 electrons

2 Complex II

Believe it or not you are already familiar

with Complex II It is none other than succinate

dehydrogenase The only enzyme of the citric acid

cycle that is an integral membrane protein This

complex is composed of four subunits 2 of which are

iron-sulfur proteins and the other two subunits

together bind FAD through a covalent link to a

histidine residue These two subunits are called

flavoprotein 2 or FP2 Complex II contains 3 Fe-S

centers 1 4Fe-4S cluster 1 3Fe-4S cluster and 1

2Fe- 2S cluster In the first step of this complex

succinate is bound and a hydride is transferred to

FAD to generate FADH2 and fumarate FADH2 then

transfers its electrons one at a time to the Fe-S

centers Thus once again FAD functions as 2 electron

acceptor and a 1 electron donor The final step of

this complex is the transfer of 2 electrons one at a

time to coenzyme Q to produce CoQH2 For complex II

the standard free energy change of the overall

reaction is too small to drive the transport of

protons across the inner mitochondrial membrane This

25

accounts for the 15 ATPrsquos generated per FADH2

compared with the 25 ATPrsquos generated per NADH

3 Complex III

This complex is also known as coenzyme Q-

cytochrome c reductase because it passes the

electrons form CoQH2 to cyt c through a very unique

electron transport pathway called the Q-cycle Shown

to the left are the porphyrins found in cytochromes

Cytochrome b contains the same iron protoporphyrin as

hemoglobin and myoglobin The c cytochromes contain

heme c through covalent attachment by cysteine

residues Cytochrome a is found in two forms in

complex IV In complex III we find two b-type

cytochromes and one c-type cytochrome Complex III is

complex and we have a crystal structure

4 Complex IV

Complex IV is also known as cytochrome c oxidase

because it accepts the electrons from cytochrome c

and directs them towards the four electron reduction

of O2 to form 2 molecules of H2O

4 cyt c (Fe2+) + 4 H+ + O2 4 cyt c (Fe3+) +

2H2O

26

Cytochrome c oxidase contains 2 heme

centerscytochrome a and cytochrome a3 and two copper

proteins Each of the protein bound coppers are

associated with one of the cytochromes The copper

sites are called CuA and CuB CuA is associated with

cytochrome a and is shown to the left CuB is

associated with cytochrome a3 The copper sites

function as 1 electron carriers cycling between the

cuprous state Cu+ and the cupric state Cu2+ Just

like iron containing proteins they transfer

electrons one at a time Cytochrome c is bound on the

P-face of the membrane and transfers its electron to

CuA The oxidized cytochrome c dissociates CuA then

transfers the electron to cytochrome a The protein

bound CuA and the iron bound in cytochrome a are 15 Aring

apart In contrast the CuB and the iron bound in

cytochrome a3 are very close to each other forming a

binuclear metal center shown below

27

Electron transport chains are associated with

membranes such as the mitochondrial membrane in

eukaryotic organisms the thylakoid membrane of

photosynthetic organisms and the plasma membrane of

prokaryotic organisms While some of the details

differ electron transport chains in all organisms

operate on the same basic principles

Excited electrons are brought to the electron

transport chain by electron carriers such as NADH

and FADH2 In photosynthetic organisms the electrons

are donated by water and excited by light energy

The electron donor is oxidized as the electrons are

passed to the first protein complex in the chain As

it accepts the electrons the protein complex is

reduced Next the electrons are passed from the

28

first protein complex which becomes oxidized to

the second protein complex which becomes reduced

Each of these redox reactions is slightly exergonic

or energy releasing Some of this energy is used to

move protons across the membrane against their

concentration gradient With each successive

transfer of electrons the original excited

electrons lose some of their energy After several

successive redox reactions the electrons are

donated to a terminal electron acceptor In aerobic

respiration the terminal electron acceptor is

oxygen In photosynthesis the terminal electron

acceptor is a compound called NADP

The energy in the excited electrons has been

transformed into an electrochemical gradient

represented by the high concentration of protons on

one side of the membrane These protons can diffuse

back across the membrane by passing through a

protein complex called ATP synthase As these

protons pass through ATP synthase down their

concentration gradient they provide the energy

needed to make ATP from ADP and inorganic phosphate

within the cell

29

CHAPTER III

CLOSING

A Conclution

When a hexose is completely oxidized to CO2

and H2O using these three processes Glycolysis

yields two ATP and two NADH per hexose used Each

such NADH oxidized by the electron transport system

30

yields two ATP as dscribed above so glycolysis

contributes a total of six ATP per hexose The Krebs

cycle contributes two ATP per hexose or per two

pyruvates when succinyl CoA is cleaved to succinate

and CoASH This cycle also produces eight NADH per

hexose within the mitochondrial matrix by oxidative

phosporylation each of these NADH yields three ATP

or 24 per hexose Each FADH2 from the Krebs cycle

yields two ATP by oxydative phosporylation or four

per hexose (two pyruvates) The total contribution of

the Krebs cycle is then 30 Adding these 30 to the 6

from glycolysis leads to a total of 36 per hexose

completely respired by these process

B Suggestion

My greatest wish is that the readers find

this manuscript helpful in gaining the knowledge they

need for effective learning and practice and in

preparing for examination designed to test that

knowledge Reader comments and suggestions are

welcome and would be very helpful in improving our

manuscript Your contribution may assist many other

students in the coming years

31

REFERENCE

Anonim 2013 Anaerobic Respiration

httpwwwscoolcoukgcsebiologyrespiration

revise-itanaerobic-respiration Accessed

September 10th 2013

Anonim 2013 What is the difference between aerobic

and anaerobic respiration in plants

httpwwwbiologylifeeasyorg4010difference

-between-aerobic-anaerobic-respiration-plants

Accessed September 10th 2013

Campbell Mary K 2006 Biochemistry New York Thomson

Brooks Cole Inc

Dat J Capelli N Folzer H Bourgeade P Badot P-M

(2004) Sensing and signaling during

plantflooding Plant Physiology and

Biochemistry 42 273-282

Dat J Folzer H Parent C Badot P-M Capelli N

(2006) HypoxiastressCurrent Understanding and

Perspectives In Teixeira da Silva JA (Ed)

Floriculture Ornamental and Plant

Biotechnology Advances and Topical Issues (Vol

3) Global Science Books Isleworth United

Kingdompp 664-674

The subsequent steps involve the transfer of

electrons one at a time to a series of iron-sulfer

complexes that includes both 2Fe-2S and 4Fe-4S

clusters Note the importance of FMN First it

functions as a 2 electron acceptor in the hydride

transfer from NADH Second it functions as a 1

electron donor to the series of iron sulfur clusters

FMN and FAD often play crucial links between 2

electron transfer agents and 1 electron transfer

agents The final step of this complex is the

transfer of 2 electrons one at a time to coenzyme Q

CoQ like FMN and FAD can function as a 2 electron

donoracceptor and as a 1 electron donoracceptor

CoQ is a mobile electron carrier because its

isoprenoid tail makes it highly hydrophobic and

lipophillic It diffuses freely in the bilipid layer

of the inner mitochondrial membrane

The process of transferring electrons from NADH

to CoQ by complex I results in the net transport of

protons from the matrix side of the inner

mitochondrial membrane to the inter membrane space

where the H+ ions accumulate generating a proton

motive force The intermembrane space side of the

inner membrane is referred to as the P face (P

standing for positive) The matrix side of the inner

membrane is referred to as the the N face The

24

transport of electrons from NADH to CoQ is coupled to

the transport of protons across the membrane This is

an example of active transport The stiochiometry is

4 H+ transported per 2 electrons

2 Complex II

Believe it or not you are already familiar

with Complex II It is none other than succinate

dehydrogenase The only enzyme of the citric acid

cycle that is an integral membrane protein This

complex is composed of four subunits 2 of which are

iron-sulfur proteins and the other two subunits

together bind FAD through a covalent link to a

histidine residue These two subunits are called

flavoprotein 2 or FP2 Complex II contains 3 Fe-S

centers 1 4Fe-4S cluster 1 3Fe-4S cluster and 1

2Fe- 2S cluster In the first step of this complex

succinate is bound and a hydride is transferred to

FAD to generate FADH2 and fumarate FADH2 then

transfers its electrons one at a time to the Fe-S

centers Thus once again FAD functions as 2 electron

acceptor and a 1 electron donor The final step of

this complex is the transfer of 2 electrons one at a

time to coenzyme Q to produce CoQH2 For complex II

the standard free energy change of the overall

reaction is too small to drive the transport of

protons across the inner mitochondrial membrane This

25

accounts for the 15 ATPrsquos generated per FADH2

compared with the 25 ATPrsquos generated per NADH

3 Complex III

This complex is also known as coenzyme Q-

cytochrome c reductase because it passes the

electrons form CoQH2 to cyt c through a very unique

electron transport pathway called the Q-cycle Shown

to the left are the porphyrins found in cytochromes

Cytochrome b contains the same iron protoporphyrin as

hemoglobin and myoglobin The c cytochromes contain

heme c through covalent attachment by cysteine

residues Cytochrome a is found in two forms in

complex IV In complex III we find two b-type

cytochromes and one c-type cytochrome Complex III is

complex and we have a crystal structure

4 Complex IV

Complex IV is also known as cytochrome c oxidase

because it accepts the electrons from cytochrome c

and directs them towards the four electron reduction

of O2 to form 2 molecules of H2O

4 cyt c (Fe2+) + 4 H+ + O2 4 cyt c (Fe3+) +

2H2O

26

Cytochrome c oxidase contains 2 heme

centerscytochrome a and cytochrome a3 and two copper

proteins Each of the protein bound coppers are

associated with one of the cytochromes The copper

sites are called CuA and CuB CuA is associated with

cytochrome a and is shown to the left CuB is

associated with cytochrome a3 The copper sites

function as 1 electron carriers cycling between the

cuprous state Cu+ and the cupric state Cu2+ Just

like iron containing proteins they transfer

electrons one at a time Cytochrome c is bound on the

P-face of the membrane and transfers its electron to

CuA The oxidized cytochrome c dissociates CuA then

transfers the electron to cytochrome a The protein

bound CuA and the iron bound in cytochrome a are 15 Aring

apart In contrast the CuB and the iron bound in

cytochrome a3 are very close to each other forming a

binuclear metal center shown below

27

Electron transport chains are associated with

membranes such as the mitochondrial membrane in

eukaryotic organisms the thylakoid membrane of

photosynthetic organisms and the plasma membrane of

prokaryotic organisms While some of the details

differ electron transport chains in all organisms

operate on the same basic principles

Excited electrons are brought to the electron

transport chain by electron carriers such as NADH

and FADH2 In photosynthetic organisms the electrons

are donated by water and excited by light energy

The electron donor is oxidized as the electrons are

passed to the first protein complex in the chain As

it accepts the electrons the protein complex is

reduced Next the electrons are passed from the

28

first protein complex which becomes oxidized to

the second protein complex which becomes reduced

Each of these redox reactions is slightly exergonic

or energy releasing Some of this energy is used to

move protons across the membrane against their

concentration gradient With each successive

transfer of electrons the original excited

electrons lose some of their energy After several

successive redox reactions the electrons are

donated to a terminal electron acceptor In aerobic

respiration the terminal electron acceptor is

oxygen In photosynthesis the terminal electron

acceptor is a compound called NADP

The energy in the excited electrons has been

transformed into an electrochemical gradient

represented by the high concentration of protons on

one side of the membrane These protons can diffuse

back across the membrane by passing through a

protein complex called ATP synthase As these

protons pass through ATP synthase down their

concentration gradient they provide the energy

needed to make ATP from ADP and inorganic phosphate

within the cell

29

CHAPTER III

CLOSING

A Conclution

When a hexose is completely oxidized to CO2

and H2O using these three processes Glycolysis

yields two ATP and two NADH per hexose used Each

such NADH oxidized by the electron transport system

30

yields two ATP as dscribed above so glycolysis

contributes a total of six ATP per hexose The Krebs

cycle contributes two ATP per hexose or per two

pyruvates when succinyl CoA is cleaved to succinate

and CoASH This cycle also produces eight NADH per

hexose within the mitochondrial matrix by oxidative

phosporylation each of these NADH yields three ATP

or 24 per hexose Each FADH2 from the Krebs cycle

yields two ATP by oxydative phosporylation or four

per hexose (two pyruvates) The total contribution of

the Krebs cycle is then 30 Adding these 30 to the 6

from glycolysis leads to a total of 36 per hexose

completely respired by these process

B Suggestion

My greatest wish is that the readers find

this manuscript helpful in gaining the knowledge they

need for effective learning and practice and in

preparing for examination designed to test that

knowledge Reader comments and suggestions are

welcome and would be very helpful in improving our

manuscript Your contribution may assist many other

students in the coming years

31

REFERENCE

Anonim 2013 Anaerobic Respiration

httpwwwscoolcoukgcsebiologyrespiration

revise-itanaerobic-respiration Accessed

September 10th 2013

Anonim 2013 What is the difference between aerobic

and anaerobic respiration in plants

httpwwwbiologylifeeasyorg4010difference

-between-aerobic-anaerobic-respiration-plants

Accessed September 10th 2013

Campbell Mary K 2006 Biochemistry New York Thomson

Brooks Cole Inc

Dat J Capelli N Folzer H Bourgeade P Badot P-M

(2004) Sensing and signaling during

plantflooding Plant Physiology and

Biochemistry 42 273-282

Dat J Folzer H Parent C Badot P-M Capelli N

(2006) HypoxiastressCurrent Understanding and

Perspectives In Teixeira da Silva JA (Ed)

Floriculture Ornamental and Plant

Biotechnology Advances and Topical Issues (Vol

3) Global Science Books Isleworth United

Kingdompp 664-674

transport of electrons from NADH to CoQ is coupled to

the transport of protons across the membrane This is

an example of active transport The stiochiometry is

4 H+ transported per 2 electrons

2 Complex II

Believe it or not you are already familiar

with Complex II It is none other than succinate

dehydrogenase The only enzyme of the citric acid

cycle that is an integral membrane protein This

complex is composed of four subunits 2 of which are

iron-sulfur proteins and the other two subunits

together bind FAD through a covalent link to a

histidine residue These two subunits are called

flavoprotein 2 or FP2 Complex II contains 3 Fe-S

centers 1 4Fe-4S cluster 1 3Fe-4S cluster and 1

2Fe- 2S cluster In the first step of this complex

succinate is bound and a hydride is transferred to

FAD to generate FADH2 and fumarate FADH2 then

transfers its electrons one at a time to the Fe-S

centers Thus once again FAD functions as 2 electron

acceptor and a 1 electron donor The final step of

this complex is the transfer of 2 electrons one at a

time to coenzyme Q to produce CoQH2 For complex II

the standard free energy change of the overall

reaction is too small to drive the transport of

protons across the inner mitochondrial membrane This

25

accounts for the 15 ATPrsquos generated per FADH2

compared with the 25 ATPrsquos generated per NADH

3 Complex III

This complex is also known as coenzyme Q-

cytochrome c reductase because it passes the

electrons form CoQH2 to cyt c through a very unique

electron transport pathway called the Q-cycle Shown

to the left are the porphyrins found in cytochromes

Cytochrome b contains the same iron protoporphyrin as

hemoglobin and myoglobin The c cytochromes contain

heme c through covalent attachment by cysteine

residues Cytochrome a is found in two forms in

complex IV In complex III we find two b-type

cytochromes and one c-type cytochrome Complex III is

complex and we have a crystal structure

4 Complex IV

Complex IV is also known as cytochrome c oxidase

because it accepts the electrons from cytochrome c

and directs them towards the four electron reduction

of O2 to form 2 molecules of H2O

4 cyt c (Fe2+) + 4 H+ + O2 4 cyt c (Fe3+) +

2H2O

26

Cytochrome c oxidase contains 2 heme

centerscytochrome a and cytochrome a3 and two copper

proteins Each of the protein bound coppers are

associated with one of the cytochromes The copper

sites are called CuA and CuB CuA is associated with

cytochrome a and is shown to the left CuB is

associated with cytochrome a3 The copper sites

function as 1 electron carriers cycling between the

cuprous state Cu+ and the cupric state Cu2+ Just

like iron containing proteins they transfer

electrons one at a time Cytochrome c is bound on the

P-face of the membrane and transfers its electron to

CuA The oxidized cytochrome c dissociates CuA then

transfers the electron to cytochrome a The protein

bound CuA and the iron bound in cytochrome a are 15 Aring

apart In contrast the CuB and the iron bound in

cytochrome a3 are very close to each other forming a

binuclear metal center shown below

27

Electron transport chains are associated with

membranes such as the mitochondrial membrane in

eukaryotic organisms the thylakoid membrane of

photosynthetic organisms and the plasma membrane of

prokaryotic organisms While some of the details

differ electron transport chains in all organisms

operate on the same basic principles

Excited electrons are brought to the electron

transport chain by electron carriers such as NADH

and FADH2 In photosynthetic organisms the electrons

are donated by water and excited by light energy

The electron donor is oxidized as the electrons are

passed to the first protein complex in the chain As

it accepts the electrons the protein complex is

reduced Next the electrons are passed from the

28

first protein complex which becomes oxidized to

the second protein complex which becomes reduced

Each of these redox reactions is slightly exergonic

or energy releasing Some of this energy is used to

move protons across the membrane against their

concentration gradient With each successive

transfer of electrons the original excited

electrons lose some of their energy After several

successive redox reactions the electrons are

donated to a terminal electron acceptor In aerobic

respiration the terminal electron acceptor is

oxygen In photosynthesis the terminal electron

acceptor is a compound called NADP

The energy in the excited electrons has been

transformed into an electrochemical gradient

represented by the high concentration of protons on

one side of the membrane These protons can diffuse

back across the membrane by passing through a

protein complex called ATP synthase As these

protons pass through ATP synthase down their

concentration gradient they provide the energy

needed to make ATP from ADP and inorganic phosphate

within the cell

29

CHAPTER III

CLOSING

A Conclution

When a hexose is completely oxidized to CO2

and H2O using these three processes Glycolysis

yields two ATP and two NADH per hexose used Each

such NADH oxidized by the electron transport system

30

yields two ATP as dscribed above so glycolysis

contributes a total of six ATP per hexose The Krebs

cycle contributes two ATP per hexose or per two

pyruvates when succinyl CoA is cleaved to succinate

and CoASH This cycle also produces eight NADH per

hexose within the mitochondrial matrix by oxidative

phosporylation each of these NADH yields three ATP

or 24 per hexose Each FADH2 from the Krebs cycle

yields two ATP by oxydative phosporylation or four

per hexose (two pyruvates) The total contribution of

the Krebs cycle is then 30 Adding these 30 to the 6

from glycolysis leads to a total of 36 per hexose

completely respired by these process

B Suggestion

My greatest wish is that the readers find

this manuscript helpful in gaining the knowledge they

need for effective learning and practice and in

preparing for examination designed to test that

knowledge Reader comments and suggestions are

welcome and would be very helpful in improving our

manuscript Your contribution may assist many other

students in the coming years

31

REFERENCE

Anonim 2013 Anaerobic Respiration

httpwwwscoolcoukgcsebiologyrespiration

revise-itanaerobic-respiration Accessed

September 10th 2013

Anonim 2013 What is the difference between aerobic

and anaerobic respiration in plants

httpwwwbiologylifeeasyorg4010difference

-between-aerobic-anaerobic-respiration-plants

Accessed September 10th 2013

Campbell Mary K 2006 Biochemistry New York Thomson

Brooks Cole Inc

Dat J Capelli N Folzer H Bourgeade P Badot P-M

(2004) Sensing and signaling during

plantflooding Plant Physiology and

Biochemistry 42 273-282

Dat J Folzer H Parent C Badot P-M Capelli N

(2006) HypoxiastressCurrent Understanding and

Perspectives In Teixeira da Silva JA (Ed)

Floriculture Ornamental and Plant

Biotechnology Advances and Topical Issues (Vol

3) Global Science Books Isleworth United

Kingdompp 664-674

accounts for the 15 ATPrsquos generated per FADH2

compared with the 25 ATPrsquos generated per NADH

3 Complex III

This complex is also known as coenzyme Q-

cytochrome c reductase because it passes the

electrons form CoQH2 to cyt c through a very unique

electron transport pathway called the Q-cycle Shown

to the left are the porphyrins found in cytochromes

Cytochrome b contains the same iron protoporphyrin as

hemoglobin and myoglobin The c cytochromes contain

heme c through covalent attachment by cysteine

residues Cytochrome a is found in two forms in

complex IV In complex III we find two b-type

cytochromes and one c-type cytochrome Complex III is

complex and we have a crystal structure

4 Complex IV

Complex IV is also known as cytochrome c oxidase

because it accepts the electrons from cytochrome c

and directs them towards the four electron reduction

of O2 to form 2 molecules of H2O

4 cyt c (Fe2+) + 4 H+ + O2 4 cyt c (Fe3+) +

2H2O

26

Cytochrome c oxidase contains 2 heme

centerscytochrome a and cytochrome a3 and two copper

proteins Each of the protein bound coppers are

associated with one of the cytochromes The copper

sites are called CuA and CuB CuA is associated with

cytochrome a and is shown to the left CuB is

associated with cytochrome a3 The copper sites

function as 1 electron carriers cycling between the

cuprous state Cu+ and the cupric state Cu2+ Just

like iron containing proteins they transfer

electrons one at a time Cytochrome c is bound on the

P-face of the membrane and transfers its electron to

CuA The oxidized cytochrome c dissociates CuA then

transfers the electron to cytochrome a The protein

bound CuA and the iron bound in cytochrome a are 15 Aring

apart In contrast the CuB and the iron bound in

cytochrome a3 are very close to each other forming a

binuclear metal center shown below

27

Electron transport chains are associated with

membranes such as the mitochondrial membrane in

eukaryotic organisms the thylakoid membrane of

photosynthetic organisms and the plasma membrane of

prokaryotic organisms While some of the details

differ electron transport chains in all organisms

operate on the same basic principles

Excited electrons are brought to the electron

transport chain by electron carriers such as NADH

and FADH2 In photosynthetic organisms the electrons

are donated by water and excited by light energy

The electron donor is oxidized as the electrons are

passed to the first protein complex in the chain As

it accepts the electrons the protein complex is

reduced Next the electrons are passed from the

28

first protein complex which becomes oxidized to

the second protein complex which becomes reduced

Each of these redox reactions is slightly exergonic

or energy releasing Some of this energy is used to

move protons across the membrane against their

concentration gradient With each successive

transfer of electrons the original excited

electrons lose some of their energy After several

successive redox reactions the electrons are

donated to a terminal electron acceptor In aerobic

respiration the terminal electron acceptor is

oxygen In photosynthesis the terminal electron

acceptor is a compound called NADP

The energy in the excited electrons has been

transformed into an electrochemical gradient

represented by the high concentration of protons on

one side of the membrane These protons can diffuse

back across the membrane by passing through a

protein complex called ATP synthase As these

protons pass through ATP synthase down their

concentration gradient they provide the energy

needed to make ATP from ADP and inorganic phosphate

within the cell

29

CHAPTER III

CLOSING

A Conclution

When a hexose is completely oxidized to CO2

and H2O using these three processes Glycolysis

yields two ATP and two NADH per hexose used Each

such NADH oxidized by the electron transport system

30

yields two ATP as dscribed above so glycolysis

contributes a total of six ATP per hexose The Krebs

cycle contributes two ATP per hexose or per two

pyruvates when succinyl CoA is cleaved to succinate

and CoASH This cycle also produces eight NADH per

hexose within the mitochondrial matrix by oxidative

phosporylation each of these NADH yields three ATP

or 24 per hexose Each FADH2 from the Krebs cycle

yields two ATP by oxydative phosporylation or four

per hexose (two pyruvates) The total contribution of

the Krebs cycle is then 30 Adding these 30 to the 6

from glycolysis leads to a total of 36 per hexose

completely respired by these process

B Suggestion

My greatest wish is that the readers find

this manuscript helpful in gaining the knowledge they

need for effective learning and practice and in

preparing for examination designed to test that

knowledge Reader comments and suggestions are

welcome and would be very helpful in improving our

manuscript Your contribution may assist many other

students in the coming years

31

REFERENCE

Anonim 2013 Anaerobic Respiration

httpwwwscoolcoukgcsebiologyrespiration

revise-itanaerobic-respiration Accessed

September 10th 2013

Anonim 2013 What is the difference between aerobic

and anaerobic respiration in plants

httpwwwbiologylifeeasyorg4010difference

-between-aerobic-anaerobic-respiration-plants

Accessed September 10th 2013

Campbell Mary K 2006 Biochemistry New York Thomson

Brooks Cole Inc

Dat J Capelli N Folzer H Bourgeade P Badot P-M

(2004) Sensing and signaling during

plantflooding Plant Physiology and

Biochemistry 42 273-282

Dat J Folzer H Parent C Badot P-M Capelli N

(2006) HypoxiastressCurrent Understanding and

Perspectives In Teixeira da Silva JA (Ed)

Floriculture Ornamental and Plant

Biotechnology Advances and Topical Issues (Vol

3) Global Science Books Isleworth United

Kingdompp 664-674

Cytochrome c oxidase contains 2 heme

centerscytochrome a and cytochrome a3 and two copper

proteins Each of the protein bound coppers are

associated with one of the cytochromes The copper

sites are called CuA and CuB CuA is associated with

cytochrome a and is shown to the left CuB is

associated with cytochrome a3 The copper sites

function as 1 electron carriers cycling between the

cuprous state Cu+ and the cupric state Cu2+ Just

like iron containing proteins they transfer

electrons one at a time Cytochrome c is bound on the

P-face of the membrane and transfers its electron to

CuA The oxidized cytochrome c dissociates CuA then

transfers the electron to cytochrome a The protein

bound CuA and the iron bound in cytochrome a are 15 Aring

apart In contrast the CuB and the iron bound in

cytochrome a3 are very close to each other forming a

binuclear metal center shown below

27

Electron transport chains are associated with

membranes such as the mitochondrial membrane in

eukaryotic organisms the thylakoid membrane of

photosynthetic organisms and the plasma membrane of

prokaryotic organisms While some of the details

differ electron transport chains in all organisms

operate on the same basic principles

Excited electrons are brought to the electron

transport chain by electron carriers such as NADH

and FADH2 In photosynthetic organisms the electrons

are donated by water and excited by light energy

The electron donor is oxidized as the electrons are

passed to the first protein complex in the chain As

it accepts the electrons the protein complex is

reduced Next the electrons are passed from the

28

first protein complex which becomes oxidized to

the second protein complex which becomes reduced

Each of these redox reactions is slightly exergonic

or energy releasing Some of this energy is used to

move protons across the membrane against their

concentration gradient With each successive

transfer of electrons the original excited

electrons lose some of their energy After several

successive redox reactions the electrons are

donated to a terminal electron acceptor In aerobic

respiration the terminal electron acceptor is

oxygen In photosynthesis the terminal electron

acceptor is a compound called NADP

The energy in the excited electrons has been

transformed into an electrochemical gradient

represented by the high concentration of protons on

one side of the membrane These protons can diffuse

back across the membrane by passing through a

protein complex called ATP synthase As these

protons pass through ATP synthase down their

concentration gradient they provide the energy

needed to make ATP from ADP and inorganic phosphate

within the cell

29

CHAPTER III

CLOSING

A Conclution

When a hexose is completely oxidized to CO2

and H2O using these three processes Glycolysis

yields two ATP and two NADH per hexose used Each

such NADH oxidized by the electron transport system

30

yields two ATP as dscribed above so glycolysis

contributes a total of six ATP per hexose The Krebs

cycle contributes two ATP per hexose or per two

pyruvates when succinyl CoA is cleaved to succinate

and CoASH This cycle also produces eight NADH per

hexose within the mitochondrial matrix by oxidative

phosporylation each of these NADH yields three ATP

or 24 per hexose Each FADH2 from the Krebs cycle

yields two ATP by oxydative phosporylation or four

per hexose (two pyruvates) The total contribution of

the Krebs cycle is then 30 Adding these 30 to the 6

from glycolysis leads to a total of 36 per hexose

completely respired by these process

B Suggestion

My greatest wish is that the readers find

this manuscript helpful in gaining the knowledge they

need for effective learning and practice and in

preparing for examination designed to test that

knowledge Reader comments and suggestions are

welcome and would be very helpful in improving our

manuscript Your contribution may assist many other

students in the coming years

31

REFERENCE

Anonim 2013 Anaerobic Respiration

httpwwwscoolcoukgcsebiologyrespiration

revise-itanaerobic-respiration Accessed

September 10th 2013

Anonim 2013 What is the difference between aerobic

and anaerobic respiration in plants

httpwwwbiologylifeeasyorg4010difference

-between-aerobic-anaerobic-respiration-plants

Accessed September 10th 2013

Campbell Mary K 2006 Biochemistry New York Thomson

Brooks Cole Inc

Dat J Capelli N Folzer H Bourgeade P Badot P-M

(2004) Sensing and signaling during

plantflooding Plant Physiology and

Biochemistry 42 273-282

Dat J Folzer H Parent C Badot P-M Capelli N

(2006) HypoxiastressCurrent Understanding and

Perspectives In Teixeira da Silva JA (Ed)

Floriculture Ornamental and Plant

Biotechnology Advances and Topical Issues (Vol

3) Global Science Books Isleworth United

Kingdompp 664-674

Electron transport chains are associated with

membranes such as the mitochondrial membrane in

eukaryotic organisms the thylakoid membrane of

photosynthetic organisms and the plasma membrane of

prokaryotic organisms While some of the details

differ electron transport chains in all organisms

operate on the same basic principles

Excited electrons are brought to the electron

transport chain by electron carriers such as NADH

and FADH2 In photosynthetic organisms the electrons

are donated by water and excited by light energy

The electron donor is oxidized as the electrons are

passed to the first protein complex in the chain As

it accepts the electrons the protein complex is

reduced Next the electrons are passed from the

28

first protein complex which becomes oxidized to

the second protein complex which becomes reduced

Each of these redox reactions is slightly exergonic

or energy releasing Some of this energy is used to

move protons across the membrane against their

concentration gradient With each successive

transfer of electrons the original excited

electrons lose some of their energy After several

successive redox reactions the electrons are

donated to a terminal electron acceptor In aerobic

respiration the terminal electron acceptor is

oxygen In photosynthesis the terminal electron

acceptor is a compound called NADP

The energy in the excited electrons has been

transformed into an electrochemical gradient

represented by the high concentration of protons on

one side of the membrane These protons can diffuse

back across the membrane by passing through a

protein complex called ATP synthase As these

protons pass through ATP synthase down their

concentration gradient they provide the energy

needed to make ATP from ADP and inorganic phosphate

within the cell

29

CHAPTER III

CLOSING

A Conclution

When a hexose is completely oxidized to CO2

and H2O using these three processes Glycolysis

yields two ATP and two NADH per hexose used Each

such NADH oxidized by the electron transport system

30

yields two ATP as dscribed above so glycolysis

contributes a total of six ATP per hexose The Krebs

cycle contributes two ATP per hexose or per two

pyruvates when succinyl CoA is cleaved to succinate

and CoASH This cycle also produces eight NADH per

hexose within the mitochondrial matrix by oxidative

phosporylation each of these NADH yields three ATP

or 24 per hexose Each FADH2 from the Krebs cycle

yields two ATP by oxydative phosporylation or four

per hexose (two pyruvates) The total contribution of

the Krebs cycle is then 30 Adding these 30 to the 6

from glycolysis leads to a total of 36 per hexose

completely respired by these process

B Suggestion

My greatest wish is that the readers find

this manuscript helpful in gaining the knowledge they

need for effective learning and practice and in

preparing for examination designed to test that

knowledge Reader comments and suggestions are

welcome and would be very helpful in improving our

manuscript Your contribution may assist many other

students in the coming years

31

REFERENCE

Anonim 2013 Anaerobic Respiration

httpwwwscoolcoukgcsebiologyrespiration

revise-itanaerobic-respiration Accessed

September 10th 2013

Anonim 2013 What is the difference between aerobic

and anaerobic respiration in plants

httpwwwbiologylifeeasyorg4010difference

-between-aerobic-anaerobic-respiration-plants

Accessed September 10th 2013

Campbell Mary K 2006 Biochemistry New York Thomson

Brooks Cole Inc

Dat J Capelli N Folzer H Bourgeade P Badot P-M

(2004) Sensing and signaling during

plantflooding Plant Physiology and

Biochemistry 42 273-282

Dat J Folzer H Parent C Badot P-M Capelli N

(2006) HypoxiastressCurrent Understanding and

Perspectives In Teixeira da Silva JA (Ed)

Floriculture Ornamental and Plant

Biotechnology Advances and Topical Issues (Vol

3) Global Science Books Isleworth United

Kingdompp 664-674

first protein complex which becomes oxidized to

the second protein complex which becomes reduced

Each of these redox reactions is slightly exergonic

or energy releasing Some of this energy is used to

move protons across the membrane against their

concentration gradient With each successive

transfer of electrons the original excited

electrons lose some of their energy After several

successive redox reactions the electrons are

donated to a terminal electron acceptor In aerobic

respiration the terminal electron acceptor is

oxygen In photosynthesis the terminal electron

acceptor is a compound called NADP

The energy in the excited electrons has been

transformed into an electrochemical gradient

represented by the high concentration of protons on

one side of the membrane These protons can diffuse

back across the membrane by passing through a

protein complex called ATP synthase As these

protons pass through ATP synthase down their

concentration gradient they provide the energy

needed to make ATP from ADP and inorganic phosphate

within the cell

29

CHAPTER III

CLOSING

A Conclution

When a hexose is completely oxidized to CO2

and H2O using these three processes Glycolysis

yields two ATP and two NADH per hexose used Each

such NADH oxidized by the electron transport system

30

yields two ATP as dscribed above so glycolysis

contributes a total of six ATP per hexose The Krebs

cycle contributes two ATP per hexose or per two

pyruvates when succinyl CoA is cleaved to succinate

and CoASH This cycle also produces eight NADH per

hexose within the mitochondrial matrix by oxidative

phosporylation each of these NADH yields three ATP

or 24 per hexose Each FADH2 from the Krebs cycle

yields two ATP by oxydative phosporylation or four

per hexose (two pyruvates) The total contribution of

the Krebs cycle is then 30 Adding these 30 to the 6

from glycolysis leads to a total of 36 per hexose

completely respired by these process

B Suggestion

My greatest wish is that the readers find

this manuscript helpful in gaining the knowledge they

need for effective learning and practice and in

preparing for examination designed to test that

knowledge Reader comments and suggestions are

welcome and would be very helpful in improving our

manuscript Your contribution may assist many other

students in the coming years

31

REFERENCE

Anonim 2013 Anaerobic Respiration

httpwwwscoolcoukgcsebiologyrespiration

revise-itanaerobic-respiration Accessed

September 10th 2013

Anonim 2013 What is the difference between aerobic

and anaerobic respiration in plants

httpwwwbiologylifeeasyorg4010difference

-between-aerobic-anaerobic-respiration-plants

Accessed September 10th 2013

Campbell Mary K 2006 Biochemistry New York Thomson

Brooks Cole Inc

Dat J Capelli N Folzer H Bourgeade P Badot P-M

(2004) Sensing and signaling during

plantflooding Plant Physiology and

Biochemistry 42 273-282

Dat J Folzer H Parent C Badot P-M Capelli N

(2006) HypoxiastressCurrent Understanding and

Perspectives In Teixeira da Silva JA (Ed)

Floriculture Ornamental and Plant

Biotechnology Advances and Topical Issues (Vol

3) Global Science Books Isleworth United

Kingdompp 664-674

CHAPTER III

CLOSING

A Conclution

When a hexose is completely oxidized to CO2

and H2O using these three processes Glycolysis

yields two ATP and two NADH per hexose used Each

such NADH oxidized by the electron transport system

30

yields two ATP as dscribed above so glycolysis

contributes a total of six ATP per hexose The Krebs

cycle contributes two ATP per hexose or per two

pyruvates when succinyl CoA is cleaved to succinate

and CoASH This cycle also produces eight NADH per

hexose within the mitochondrial matrix by oxidative

phosporylation each of these NADH yields three ATP

or 24 per hexose Each FADH2 from the Krebs cycle

yields two ATP by oxydative phosporylation or four

per hexose (two pyruvates) The total contribution of

the Krebs cycle is then 30 Adding these 30 to the 6

from glycolysis leads to a total of 36 per hexose

completely respired by these process

B Suggestion

My greatest wish is that the readers find

this manuscript helpful in gaining the knowledge they

need for effective learning and practice and in

preparing for examination designed to test that

knowledge Reader comments and suggestions are

welcome and would be very helpful in improving our

manuscript Your contribution may assist many other

students in the coming years

31

REFERENCE

Anonim 2013 Anaerobic Respiration

httpwwwscoolcoukgcsebiologyrespiration

revise-itanaerobic-respiration Accessed

September 10th 2013

Anonim 2013 What is the difference between aerobic

and anaerobic respiration in plants

httpwwwbiologylifeeasyorg4010difference

-between-aerobic-anaerobic-respiration-plants

Accessed September 10th 2013

Campbell Mary K 2006 Biochemistry New York Thomson

Brooks Cole Inc

Dat J Capelli N Folzer H Bourgeade P Badot P-M

(2004) Sensing and signaling during

plantflooding Plant Physiology and

Biochemistry 42 273-282

Dat J Folzer H Parent C Badot P-M Capelli N

(2006) HypoxiastressCurrent Understanding and

Perspectives In Teixeira da Silva JA (Ed)

Floriculture Ornamental and Plant

Biotechnology Advances and Topical Issues (Vol

3) Global Science Books Isleworth United

Kingdompp 664-674

yields two ATP as dscribed above so glycolysis

contributes a total of six ATP per hexose The Krebs

cycle contributes two ATP per hexose or per two

pyruvates when succinyl CoA is cleaved to succinate

and CoASH This cycle also produces eight NADH per

hexose within the mitochondrial matrix by oxidative

phosporylation each of these NADH yields three ATP

or 24 per hexose Each FADH2 from the Krebs cycle

yields two ATP by oxydative phosporylation or four

per hexose (two pyruvates) The total contribution of

the Krebs cycle is then 30 Adding these 30 to the 6

from glycolysis leads to a total of 36 per hexose

completely respired by these process

B Suggestion

My greatest wish is that the readers find

this manuscript helpful in gaining the knowledge they

need for effective learning and practice and in

preparing for examination designed to test that

knowledge Reader comments and suggestions are

welcome and would be very helpful in improving our

manuscript Your contribution may assist many other

students in the coming years

31

REFERENCE

Anonim 2013 Anaerobic Respiration

httpwwwscoolcoukgcsebiologyrespiration

revise-itanaerobic-respiration Accessed

September 10th 2013

Anonim 2013 What is the difference between aerobic

and anaerobic respiration in plants

httpwwwbiologylifeeasyorg4010difference

-between-aerobic-anaerobic-respiration-plants

Accessed September 10th 2013

Campbell Mary K 2006 Biochemistry New York Thomson

Brooks Cole Inc

Dat J Capelli N Folzer H Bourgeade P Badot P-M

(2004) Sensing and signaling during

plantflooding Plant Physiology and

Biochemistry 42 273-282

Dat J Folzer H Parent C Badot P-M Capelli N

(2006) HypoxiastressCurrent Understanding and

Perspectives In Teixeira da Silva JA (Ed)

Floriculture Ornamental and Plant

Biotechnology Advances and Topical Issues (Vol

3) Global Science Books Isleworth United

Kingdompp 664-674

REFERENCE

Anonim 2013 Anaerobic Respiration

httpwwwscoolcoukgcsebiologyrespiration

revise-itanaerobic-respiration Accessed

September 10th 2013

Anonim 2013 What is the difference between aerobic

and anaerobic respiration in plants

httpwwwbiologylifeeasyorg4010difference

-between-aerobic-anaerobic-respiration-plants

Accessed September 10th 2013

Campbell Mary K 2006 Biochemistry New York Thomson

Brooks Cole Inc

Dat J Capelli N Folzer H Bourgeade P Badot P-M

(2004) Sensing and signaling during

plantflooding Plant Physiology and

Biochemistry 42 273-282

Dat J Folzer H Parent C Badot P-M Capelli N

(2006) HypoxiastressCurrent Understanding and

Perspectives In Teixeira da Silva JA (Ed)

Floriculture Ornamental and Plant

Biotechnology Advances and Topical Issues (Vol

3) Global Science Books Isleworth United

Kingdompp 664-674

Hatta Muhammad 2011 Dunia Pertanian

httpemhattawordpresscomcategorystress

Accessed September 10th 2013

Hopkins William G 2009 Introduction to Plant Physiology

New York John Wiley amp Sons Inc

Lakitan Benyamin 2011 Dasar-Dasar Fisiologi Tumbuhan

Jakarta PT Raja Grafindo Persada

Larcher W 1980 Physiological Plant Ecology New York

Springer-Verlag

Raven 2013 Biology Fifth Edition

httphigheredmcgraw-hillcomsites983409233

9student_view0chapter39

photosynthetic_electron_transport_and_atp_synth

esishtml Accessed September 10th 2013

Salisbury Frank B 1969 Plant Physiology Third Edition

California Wadsworth Publishing Company

Subbaiah C Sachs M (2003) Molecular and cellular

adaptations of maize to flooding stress Annals

of Botany 91 119-127

Wilson CarlL1962 Botany Third EditionUnited State of

America Holt Rinch Inc