functions of food source of energy essential nutrients stored for future use metabolism is all...

Functions of food source of energy essential nutrients stored for future use

Metabolism is all the chemical reactions of the body some reactions produce the energy which

is stored in ATP that other reactions consume

all molecules will eventually be broken down and recycled or excreted from the body

26-1

Catabolic reactions breakdown complex organic compounds providing energy (exergonic) glycolysis, Krebs cycle and electron

transport Anabolic reactions synthesize complex

molecules from small molecules requiring energy (endergonic)

Exchange of energy requires use of ATP (adenosine triphosphate) molecule.

26-2

Each cell has about 1 billion ATP molecules that last for less than one minute

Over half of the energy released from ATP is converted to heat

26-3

Energy is found in the bonds between atoms

Oxidation is a decrease in the energy content of a molecule

Reduction is the increase in the energy content of a molecule

Oxidation-reduction reactions are always coupled within the body whenever a substance is oxidized,

another is almost simultaneously reduced.26-4

Biological oxidation involves the loss of electron and a proton (hydrogen atom)dehydrogenation reactions require

coenzymes to transfer hydrogen atoms to another compound

common coenzymes of living cells that carry H+

NAD (nicotinamide adenine dinucleotide ) NADP (nicotinamide adenine dinucleotide phosphate ) FAD (flavin adenine dinucleotide )

NAD+ + 2 H NADH + H+

Biological reduction is the addition of electron and a proton (hydrogen atom) to a moleculeincrease in potential energy of the molecule

26-5

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Dietary carbohydrate burned as fuel within hours of absorption

All oxidative carbohydrate consumption is essentially a matter of glucose catabolism

C6H12O6 + 6O2 6CO2 + 6H2O+

energy

Function of this reaction is to transfers energy from glucose to ATP not to produce carbon dioxide and water 26-7

Glucose catabolism – a series of small steps, each controlled by a separate enzyme, in which energy is released in small manageable amounts, and as much as possible, is transferred to ATP and the rest is released as heat

Three major pathways of glucose catabolism glycolysis

glucose (6C) split into 2 pyruvic acid molecules (3C)

anaerobic fermentation occurs in the absence of oxygen reduces pyruvic acid to lactic acid

aerobic respiration occurs in the presence of oxygen completely oxidizes pyruvic acid to CO2 and H2O

26-8

Phosphorylation is bond attaching 3rd

phosphate group contains stored energy

Mechanisms of phosphorylation within animals

substrate-level phosphorylation in cytosol

oxidative phosphorylation in mitochondria

in chlorophyll-containing plants or bacteria

photophosphorylation.26-9

26-10

Glucose

Glucose 6-phosphate

Glycogen Fat

Fructose 6-phosphate

Fructose 1,6-diphosphate

2 PGAL

22 NAD+

2 NADH + 2 H+

2

2 H2O

2

2

2 pyruvic acid

2 NADH + 2 H+

2 NAD+

O 2 la

ckingO

2 present

22 lactic acid

2

2

Aerobic respirationAnaerobic fermentation

5 Dephosphorylation

1 Phosphorylation

2 Priming

3 Cleavage

4 Oxidation

Pi

ATP

ATP

ATP

2 ATP

ADP

ADP

ADP

2 ADP

KeyCarbon atoms

Phosphategroups

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Phosphorylation glucose enters cell has

phosphate added - ATP used

maintains favorable concentration gradient, prevents glucose from leaving cell

Priming isomerization occurs phosphorylation further

activates molecule - ATP used

Cleavage molecule split into 2

three-carbon molecules

26-11

Oxidation removes H+

NAD+ + H NADHDephosphorylation

transfers phosphate groups to ADP to form ATP

4 ATPs produced (2 ATP used) for a net gain of 2 ATP2 ATP

produces 2 pyruvic acid Animation

26-12

http://www2.sunysuffolk.edu/jurukov/Links/Animations.htm

4 ATP are produced but 2 ATP were consumed to initiate glycolysis, so net gain is 2 ATP per glucose molecule

Some energy originally in the glucose is contained in the ATP, some in the NADH, some is lost as heat, but most of the energy remains in the pyruvic acid

End-products of glycolysis are: 2 pyruvic acid + 2 NADH + 2 ATP

26-13

Fate of pyruvic acid depends on oxygen availability

In an exercising muscle, demand for ATP > oxygen supply; ATP produced by glycolysis glycolysis can not continue without supply of

NAD+

NADH reduces pyruvic acid to lactic acid, restoring NAD+

Lactic acid travels to liver to be oxidized back to pyruvic when O2 is available (oxygen debt) then stored as glycogen or released as

glucose Fermentation is inefficient, not favored by

brain or heart26-14

Lactic acid leaves the cells that generate it enter bloodstream and transported to the liver when oxygen becomes available the liver oxidized it back to

pyruvic acid oxygen is part of the oxygen debt created by exercising

muscle

Liver can also convert lactic acid back to G6P and can: polymerize it to form glycogen for storage remove phosphate group and release free glucose into the

blood

Drawbacks of anaerobic fermentation wasteful, because most of the energy of glucose is still in the

lactic acid and has contributed no useful work lactic acid is toxic and contributes to muscle fatigue

Skeletal muscle is relatively tolerant of anaerobic fermentation, cardiac muscle less so the brain employs no anaerobic fermentation 26-15

Most ATP generated in mitochondria, which requires oxygen as final electron acceptor

In the presence of oxygen, pyruvic acid enters the mitochondria and is oxidized by aerobic respiration

Occurs in two principal steps: matrix reactions – their controlling

enzymes are in the fluid of the mitochondrial matrix

membrane reactions - whose controlling enzymes are bound to the membranes of the mitochondrial cristae

26-16

26-17

10

7

6

Pyruvic acid (C3)

CO2

NAD+

NADH + H+

Acetyl group (C2)

Acetyl-Co A

Coenzyme A

H2O

Citric acid (C6)

Oxaloacetic acid (C4) H2O

(C6)

CO2

FAD

FADH2

H2O

NADH + H+

NAD+

11

14

15

17

18

GTP GDP

12

13

16

Occurs inmitochondrialmatrix

ADP

9

8

Pi

Citricacidcycle H2O

NAD+

NADH + H+

(C4)

(C5)

NAD+

NADH + H+

CO2

(C4)

(C4)

(C4)


ATP

Pyruvic acid oxidation

Citric acid (Krebs) Cycle

Three steps prepare pyruvic acid to enter citric acid cycledecarboxylation so that a 3-

carbon compound becomes a 2-carbon compound

CO2 removed from pyruvic acid

convert that to an acetyl group (acetic acid)

NAD+ removes hydrogen atoms from the C2 compound

acetyl group binds to coenzyme A

results in acetyl-coenzyme A (acetyl-CoA)

26-18

Citric Acid Cycle acetyl-Co A (a C2 compound)

combines with a C4 to form a C6 compound (citric acid)-- start of cycle

hydrogen atoms are removed and accepted by NAD+

another CO2 is removed and the substrate becomes a five-carbon chain

previous step repeated removing another free CO2 leaving a four-carbon chain

ATP two hydrogen atoms are removed

and accepted by the coenzyme FAD two final hydrogen atoms are

removed and transferred to NAD+ reaction generates oxaloacetic acid,

which starts the cycle again 26-19

2 pyruvate + 6H2O 6CO2

2 ADP + 2 Pi 2 ATP

8 NAD+ + 8 H2 8 NADH + 8 H+

2 FAD + 2 H2 2 FADH2

Carbon atoms of glucose have all been carried away as CO2 and exhaled

Energy lost as heat, stored in 2 ATP, 8 reduced NADH, 2 FADH2 molecules of the matrix reactions and 2 NADH from glycolysis

Citric acid cycle is a source of substances for synthesis of fats and nonessential amino acids

26-21

Membrane reactions have two purposes:to further oxidize NADH and

FADH2 and transfer their energy to ATP

to regenerate NAD+ and FAD and make them available again to earlier reaction steps

Mitochondrial electron-transport chain – series of compounds that carry out this series of membrane reactionsmost bound to the inner

mitochondrial membranearranged in a precise order that

enables each one to receive a pair of electrons from the member on the left side of it.

pass electrons to member on the other side 26-22

50

40

30

20

10

Enzyme complex 1

Rel

ati

ve

free

en

erg

y (k

cal/

mo

le)

0

NADH + H+

NAD+

FADH2FAD

FMNFe-S

CoQ

Cyt b

Fe-SCyt c1

Cyt c

CuCyt a

Enzyme complex 2

Reaction progress

Enzyme complex 3

½ O2 + 2 H+

H2O

1


26-24

Figure 26.5

Cyt a3

Electron transport chain energy fuels respiratory enzyme complexes pump protons from matrix into space

between inner and outer mitochondrial membranes

creates steep electrochemical gradient for H+ across inner mitochondrial membrane

Inner membrane is permeable to H+ at channel proteins called ATP synthase

Chemiosmotic mechanism - H+ current rushing back through these ATP synthase channels drives ATP synthesis (ANIMATION)

26-25

http://www.youtube.com/watch?v=uOoHKCMAUMc&feature=related

NADH releases an electron pair to electron transport system and H+ to prime pumps enough energy to synthesize 3 ATP

FADH2 releases its electron pairs further along electron-transport system enough energy to synthesize 2 ATP

Complete aerobic oxidation of glucose to CO2 and H2O produces 36-38 ATP efficiency rating of 40% - 60% is lost as heat

26-27

26-29

2 NADH + 2 H+ 2 pyruvate Cytosol

Mitochondria

Glucose

2 NADH + 2 H+

6 NADH + 6 H+

Citric acidcycle

2 FADH2

Electron-transportchain

H2OO2

Glycolysis

Total 36–38

ATP2

ATP2

4

(net)

28–30

CO2


ATP

ATP

ATP

ATP is quickly used after it is formed it is an energy transfer molecule, not an energy storage

molecule converts the extra glucose to other compounds better

suited for energy storage (glycogen and fat)

Glycogenesis - synthesis of glycogen stimulated by insulin chains glucose monomers together

Glycogenolysis – hydrolysis of glycogen releases glucose between meals stimulated by glucagon and epinephrine only liver cells can release glucose back into blood

Gluconeogenesis - synthesis of glucose from noncarbohydrates, such as glycerol and amino acids occurs chiefly in the liver and later, kidneys if necessary

26-30

26-31Figure 26.8

Extracellular

Intracellular

Glucose 6-phosphate

Glycolysis

Key

Glycogenesis

Glycogenolysis

Glycogensynthase

Glycogenphosphorylase

Pi

Glycogen

Glucose6-phosphatase(in liver, kidney,and intestinal cells)

Bloodglucose

Hexokinase (in all cells)

Glucose1-phosphate

Pi


Triglycerides are stored in body’s adipocytes constant turnover of lipid molecules every

2 - 3 weeks released into blood, transported and either

oxidized or redeposited in other fat cells

Lipogenesis - synthesis of fat from other types of molecules amino acids and sugars used to make

fatty acids and glycerol PGAL can be converted to glycerol

26-32

Lipolysis – breaking down fat for fuel begins with the hydrolysis of a triglyceride

to glycerol and fatty acids stimulated by epinephrine, norepinephrine,

glucocorticoids, thyroid hormone, and growth hormone

glycerol easily converted to PGAL and enters the pathway of glycolysis

generates only half as much ATP as glucose

beta oxidation in the mitochondrial matrix catabolizes the fatty acid components

removes two carbon atoms at a time which bonds to coenzyme A

forms acetyl-CoA, the entry point for the citric acid cycle

a fatty acid with 16 carbons can yield 129 molecules of ATP

richer source of energy than the glucose molecule26-33

26-34

Glucose

PGAL

Glucose 6-phosphate

Key

LipogenesisLipolysis

Glycerol

Fatty acidsGlycerol

Beta oxidation

Acetyl-Co A

Storedtriglycerides

Ketone bodies β-hydroxybutyric acid Acetoacetic acid Acetone

Acetyl groups

Citricacidcycle

Pyruvicacid

Fattyacids

Newtriglycerides


Fatty acids catabolized into acetyl groups (by beta-oxidation in mitochondrial matrix) may: enter citric acid cycle as acetyl-CoA undergo ketogenesis

metabolized by liver to produce ketone bodies acetoacetic acid -hydroxybutyric acid acetone

rapid or incomplete oxidization of fats raises blood ketone levels (ketosis) and may lead to a pH imbalance (ketoacidosis)

26-35

Amino acids in the pool can be converted to others

Free amino acids also can be converted to glucose and fat or directly used as fuel

Conversions involve three processes: deamination – removal of an amino group (-NH2) amination – addition of -NH2

transamination – transfer of -NH2 from one molecule to another

As fuel - first must be deaminated (removal of -NH2) what remains is keto acid and may be converted to pyruvic

acid, acetyl-CoA, or one of the acids of the citric acid cycle during shortage of amino acids, citric acid cycle

intermediates can be aminated and converted to amino acids

in gluconeogenesis, keto acids are used to synthesis glucose26-36

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