07-chapt08 lecture anim - nassau community college · 8.1 overview of photosynthesis ... 8.2 plants...

8/30/2013

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Chapter 08

Lecture and

Animation Outline

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8.1 Overview of Photosynthesis

• Photosynthesis converts solar energy into chemical energy of carbohydrates

• Organisms that carry on photosynthesis are called autotrophs

– Plants, algae, and cyanobacteria are organisms capable of photosynthesis

• Heterotrophs are organisms that feed on other organisms

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• Autotrophs and heterotrophs use organic molecules produced by photosynthesis

• Pigments allow photosynthetic organisms to capture solar energy

• Most photosynthetic organisms contain the pigment chlorophyll

• Another common pigment group are carotenoids

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Flowering Plants as Photosynthesizers

• Photosynthesis occurs in the green parts of plants

– Particularly leaves, contain chlorophyll and other pigments

• Leaves contain mesophyll tissue specialized for photosynthesis

• Raw materials are water and CO2

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– Water is taken up by roots and transported to leaves by veins

– Carbon dioxide enters through openings in the leaves called stomata

– Light energy is absorbed by chlorophyll and other pigments in thylakoids of chloroplasts

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• Chloroplast structure

– The chloroplast and its fluid-filled interior called stroma are surrounded by a double membrane

– Thylakoids are a different membrane system

within the stroma that form flattened sacs

– Thylakoids are stacked together to from grana

– Thylakoid space is formed by a continuous connection between individual thylakoids

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cuticle

CO2

O2

stomata

upperepidermis

mesophyll

lowerepidermis

Leaf vein

Leaf cross section

Figure 8.2

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Grana

granum

stromastroma

inner membrane

outer membrane

Chloroplast

thylakoid space

thylakoid membrane

channel between

thylakoids

© Dr. George Chapman/Visuals Unlimited

Chloroplast, micrograph 37,000x

Figure 8.2

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Grana

cuticle

stomata

stromastroma

Leaf cross section

upperepidermis

mesophyll

lowerepidermis

leaf vein

CO2

O2

inner membrane

outer membrane

Chloroplast

thylakoid space

thylakoid membrane

granum

channel betweenthylakoids


© Dr. George Chapman/Visuals Unlimited

Chloroplast, micrograph 37,000x

Figure 8.2

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Photosynthetic Reaction

• Glucose and oxygen are the products of

photosynthesis

• The oxygen given off comes from water

• CO2 gains hydrogen atoms and becomes

a carbohydrate


CO2

solar energy

+ 6 H2O C6H12O6 + 6 O2pigments

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Two Sets of Reactions

• Photosynthesis consists of two sets of reactions

– Photo refers to capturing solar energy

– Synthesis refers to producing a carbohydrate

• The two sets of reactions are called the:

– Light Reactions (light-dependent)

– Calvin Cycle Reactions (light-independent)

• Nicotinamide adenine dinucleotide phosphate (NADP+) links these reactions

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CH2O

H2O CO2

ADP + P

NADP+

ATP

O2

thylakoidmembrane

stroma

Calvincycle

reactionsLight

reactions

solarenergy

NADPH


Figure 8.3

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8.2 Plants as Solar Energy

Converters• During the light reactions, different

pigments within the thylakoid membranes absorb energy

• Solar energy can be described in terms of its wavelength and energy content

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8.2 Plants as Solar Energy

Converters• The electromagnetic

spectrum extends from

very short gamma rays to very long radio waves

• White or visible light is only a small portion of the

spectrum

• Visible light is further divided into wavelengths between 380 and 750 nm

Increasing wavelength

Increasing energy

X rays UV Infrared

visible light

500 600 750

Gammarays

Micro-waves

Radiowaves

Wavelengths (nm)


380

Figure 8.4

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Visible Light

• Visible light contains various wavelengths

• The colors of visible light range from:

– Violet light

• Shortest wavelength but high energy

– Red light

• Longest wavelength but lowest energy

– Only about 42% of solar radiation that hits Earth’s atmosphere reaches the surface of Earth – most is in the visible-light range

– Higher wavelengths are screened by the ozone layer

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Visible Light

• Most photosynthetic pigments in cells are chlorophylls a and band the carotenoids

• Can absorb specific various portions of visible light

• The absorption spectrum shown in figure on the right

Wavelengths (nm)

380 500 600 750

Chlorophyll a

Chlorophyll b

carotenoidsR

ela

tive

Ab

so

rpti

on


Figure 8.5

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Visible Light

• Green light is reflected and only minimally

absorbed

– Leaves appear green

• Other plant pigments become noticeable in the fall when chlorophyll breaks down

and the other pigments are uncovered

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Light Reactions

• Light Reactions

– Take place in thylakoid membrane

– Light reactions consist of two pathways:

• Noncyclic electron pathway

• Cyclic electron pathway

– Both pathways transform solar energy to chemical energy

– Both pathways produce ATP

– Only the noncyclic pathway produces NADPH

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Noncyclic Electron Pathway• Noncyclic electron pathway, named because

electron flow is traced from water to NADP+

– Uses two photosystems (Photosystems I and II)

– A photosystem consists of a pigment complex and electron acceptors within the thylakoid membrane

– The pigment complex can be described as a

“antenna” for gathering solar energy

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Noncyclic Electron Pathway

• Noncyclic Electron Pathway begins with photosystem II (PSII)

– Pigment complex absorbs solar energy

– Energy passes from one pigment to another until it is concentrated in reaction center

• Chlorophyll a molecule

– Electrons in the reaction center chlorophyll become so energized

• Escape from the reaction center and move to a nearby electron acceptor

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• Photosystem II would disintegrate without

replacement electrons

– Electrons provided by splitting water

– Releases oxygen (O2) to atmosphere which benefits all organisms that use O2

– Hydrogen ions (H+) stay in the thylakoid space

• Contribute to formation of hydrogen ion gradient

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• In PSII, an electron acceptor receives energized electrons from the reaction center

• It sends those electrons down an electron

transport chain, (series of carriers that pass electrons from one to the other)

• Energy is released to pump hydrogen ions (H+) into thylakoid space forming gradient

• When hydrogen ions flow through ATP synthase

it makes ATP

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• PSI comes next in noncyclic electron pathway

– When the photosystem I complex absorbs solar

energy, energized electrons leave reaction center and are captured by a different electron acceptor

• Low energy PSII electrons used to replace those lost by PSI

– Electron acceptor in photosystem I passes its

electrons to NADP+and it becomes NADPH

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H2O

NADP+

H+

reaction center reaction center

Calvin cyclereactions

en

erg

y level

sunsun

Photosystem II

Photosystem I

CO2 CH2O

e–

NADPH

electron

acceptorelectron

acceptor

pigmentcomplex

2H+ O212–

e–

e– e–

e–

e–

e–

pigmentcomplex

Figure 8.6

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H2O

NADP+

H+

reaction center reaction center

Calvin cyclereactions

en

erg

y level

sunsun

Photosystem II

Photosystem I

CO2 CH2O

e–

NADPH

electron

acceptorelectron

acceptor

pigmentcomplex

2H+ O212–

e–

e– e–

e–

e–

e–

pigmentcomplex

CH2O

H2O CO2

solar

energy

ADP + P

NADP+

Light

reactions

Calv in

cycle

reactions

cycle

ATP

O2

NADPH

thylakoid

membrane

Figure 8.6

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Cyclic Electron Pathway

• Uses only photosystem I (PSI) and begins when PSI complex absorbs solar energy

– Energized electrons escape from the reaction center and travel down electron transport chain

– Released energy is stored in the form of a H+

gradient, which causes ATP production by ATP synthase

• Spent electrons return to PSI (cyclic)

• Pathway only results in ATP production

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• Energized electrons leave the photsytem I

reaction center and return to photosystem

by an electron transport chain

• ATP from cyclic electron transport

used in Calvin cycle to make carbohydrates


Calvin cycle

reactions and

other enzymatic

reactions

Pigment

complex

reaction center

CO2 CH2O

sun

Photosystem I

electron

acceptor

en

erg

y le

ve

l

e–

e–

ATP

Figure 8.7

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The Organization of the Thylakoid Membrane

• The following molecular complexes are present in the thylakoid Membrane:

– PS II

• Pigment complex and electron acceptors

• Water is split to replace energized electrons

• Oxygen (O2) is released

– Electron transport chain

• Carries electrons from PS II to PS I

• Uses energy to pump H+ from the stroma into thylakoid space

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The Organization of the Thylakoid Membrane

– PS I

• Pigment complex and electron acceptors

• Adjacent to enzyme that reduces NADP+ to NADPH

– ATP synthase complex

• Has a channel for H+ flow

• Flow drives ATP synthase to join ADP and P

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photosystem II

Stroma

Pq

H+

ATPsynthasecomplex

chemiosmosis

photosystem I

H2O +

ATP

+ ADPP

2

NADPH

O212

e–

electron transportchain

e–

e–

e– e–

thylakoidspace

NADPreductase

H+

H+

H+

H+H+

H+

H+

H+

NADP+

Figure 8.8

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photosystem II

Stroma

Pq

H+

ATPsynthasecomplex

chemiosmosis

photosystem I

granum

thylakoid membrane

thylakoid space

stroma

H2O +

ATP

+ ADPP

2

NADPH

O212

thylakoid

e–


e–

e–

e– e–

thylakoidspace

NADPreductase

H+

H+

H+

H+H+

H+

H+

H+

NADP+

Figure 8.8

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photosystem II

Stroma

Pq

H+

ATPsynthasecomplex

chemiosmosis

photosystem I

granum

thylakoid membrane

thylakoid space

stroma

H2O +

ATP

+ ADPP

2

NADPH

CH2O

H2O CO2

P

ATP

O2

O212

thylakoid

e–

thylakoid

membrane


e–

e–

e– e–

thylakoidspace

NADPreductase

H+

H+

H+

H+H+

H+

solar

energy

Calvin

cyclereactionsLight

reactions

ADP +

NADP+

NADPH

H+

H+

NADP+

Figure 8.8

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ATP Production

• ATP Production

– Thylakoid space acts as a reservoir for hydrogen ions (H+)

• H+ from water being split within thylakoid space

• Pumped in by electron transport chain

– More H+ in thylakoid space than stroma

• Electrochemical gradient

– H+ can only flow through ATP synthase

– Energy powers making ATP by chemiosmosis

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8.3 Plants as Carbon Dioxide

Fixers

• The Calvin Cycle (named after Melvin Calvin)

– Series of reactions that use CO2 from the atmosphere to produce carbohydrate

– Humans other most other organisms take in O2

and release CO2

– Includes

• Carbon dioxide fixation

• Carbon dioxide reduction

• Ribulose-1,5-bisphosphate (RuBP) regeneration

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3PG 3-phosphoglycerate

BPG 1,3-bisphosphoglycerate

Metabolites of the Calvin Cycle

P P

net gain of one G3P

Glucose

regeneration

of RuBP

intermediate

6 NADP+

6 BPG

C3

3 CO2

Other organic molecules

3 C6

x 2

3ADP + 3

These ATPmolecules wereproduced by thelight reactions.

3

ATP6 NADPH

6ADP + 6

6

ATP

RuBP ribulose-1,5-bisphosphate

G3P glyceraldehyde-3-phosphate

These ATP andNADPHmolecules wereproduced by thelight reactions.

6 3PG

C33 RuBP

C5

5 G3P

C36 G3P

C3

CO2

reduction

CO2

fixation

Calvin cycle

CH2O

stroma

O2

Lightreactions

NADP+

ATP

NADPH

Calvincycle

H2O CO2

solar

energy

ADP+ P

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Fixation of Carbon Dioxide

• Carbon dioxide fixation is the 1st step of

the Calvin cycle

– CO2 is attached to 5-carbon RuBP molecule

• This reaction occurs three times

• The result is a 6-carbon molecule that splits into two 3-carbon molecules 3-phoshoglycerate (3PG)

– RuBP Carboxylase is the enzyme that makes

this happen

• Comparatively slow enzyme so there is a lot of it

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Reduction of Carbon Dioxide

• Reduction of Carbon Dioxide

– Each 3PG molecules undergoes reduction to

G3P in two steps

– Energy and electrons needed for this reaction are supplied by ATP and NADPH (from light reaction)

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Reduction of Carbon DioxideCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

NADPH NADP+

3PG G3PBPG

ADP + P

ADP + P

ATP

As 3PG becomes G3P ATP becomes

, and NADPH becomes NADP+.

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Regeneration of RuBP

• Regeneration of RuBP

– It takes three turns of the Calvin cycle to allow

one G3P to exit

– For every three turns of Calvin Cycle, five G3P (3-carbon molecule) used

– This re-forms three RuBP (5-carbon molecule)

– 5 X 3 (carbons in G3P) = 3 X 5 (carbons in RuBP)

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Regeneration of RuBP

5 × 3 (carbons in G3P) = 3 × 5 (carbons in RuBP)


3 ATP

5 G3P 3 RuBP

3 ADP + P

As five molecules of G3P become three

molecules of RuBP, three molecules of ATP

become three molecules of ADP + P .

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Importance of the Calvin Cycle

• G3P (glyceraldehyde-3-phosphate) can be

converted to many other molecules

– These molecules meet the plant needs

• The hydrocarbon skeleton of G3P can form:

– Fatty acids and glycerol to make plant oil

– Glucose phosphate (simple sugar)

– Fructose (+ glucose = sucrose)

– Starch and cellulose

– Amino acids

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8.4 Alternate Pathways for

Photosynthesis

• C3 Photosynthesis

– The leaves of C3 plants have a different structure

and means of fixing CO2 than C4 plants

– C3 plants such as wheat, rice, oats have

mesophyll cells of leaves in parallel layers

– Bundle sheath cells around the plant veins do not

contain chloroplasts

– As a result, cells using Calvin cycle exposed to CO2

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C3 Photosynthesis

• RuBP carboxylase binds O2 as well as CO2

– When bound to O2, the enzyme undergoes

photorespiration

– Wasteful reaction because it uses O2 and releases CO2, decreasing output of Calvin cycle

– O2 concentration in leaf rises when weather is hot and dry, because plant keeps stomata

closed to conserve water

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vein

stoma

mesophyllcells

bundle sheathcell

a. C3 Plant


Calvincycle

CO2

G3P

RuBP

mesophyll cell

3PG(C3)

a. CO2 fixation in a C3 plant, tuplip

© The McGraw-Hill Companies, Inc./Evelyn Jo Johnson, photographer

Figure 8.10Figure 8.11

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C4 Photosynthesis

• C4 plants, such as sugarcane and corn, the mesophyll cells are arranged in concentric rings around the bundle sheath cells

– They also contain chloroplasts

– In the mesophyll cells, CO2 is initially fixed into a 4-carbon molecule

– The 4-carbon molecule is later broken down into a 3-carbon molecule and CO2

– CO2 enters the Calvin cycle

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C4 Photosynthesis

• C4 Pathway

– C4 plants tend to be found in hot, dry climates

– In these climates, stomata tend to close to conserve water

– Oxygen then builds-up in the leaves

– But, RuBP carboxylase is not exposed to this O2 in C4

plants and photorespiration does not occur

– Instead, in C4 plants, the CO2 is delivered to the Calvin cycle, which is located in bundle sheath cells that are sheltered from the leaf air spaces

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vein

stoma

mesophyllcells

bundle sheathcell

b. C4 Plant

CO2

CO2

C4

G3P

Calvincycle

b. CO2 fixation in a C4 plant, corn

mesophyllcell

bundlesheathcell


© Corbis RF

Figure 8.11 Figure 8.10

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C4 Photosynthesis

• When the weather is moderate, C3 plants ordinarily have the advantage.

• When the weather is hot and dry, C4 plants have

the advantage, and can be expected to predominate.

• In the early summer, C3 plants such as Kentucky bluegrass predominate in lawns in the cooler parts

of the United States, but by midsummer, crabgrass, a C4 plant, begins to take over.

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CAM Photosynthesis

• CAM Pathway

– This pathway is prevalent among most succulent

plants that grow in deserts, including the cacti.

– CAM plants partition carbon fixation according to time.

• During the night, CAM plants fix CO2, forming C4

molecules.

• The C4 molecules are stored in large vacuoles.

• During daylight, C4 molecules release CO2 to the Calvin cycle.

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CO2

CO2

C4

G3P

night

day

c. CO2 fixation in a CAM plant, pineapple

Calvin

cycle

© S. Alden/PhotoLink/Getty RF Figure 8.10


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8.5 Photosynthesis Versus Cellular Respiration

• Both plant and animal cells carry out cellular respiration.

– Occurs in mitochondria

– Breaks glucose down

– Utilizes O2 and gives off CO2

• Plant cells photosynthesize, but animal cells do not.

– Occurs in chloroplasts

– Builds glucose

– Utilizes CO2 and gives off O2

• Both processes utilize an electron transport chain and chemiosmosis for ATP production.

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Thylakoidmembrane H2O O2

ADP ATP

solarenergy

H2O CO2

Lightreactions

ADP + P

Calvincycle

reactions

NADP+

NADPH

thylakoidmembrane

O2 CH2O

stroma

Stroma NADPH NADP+

CO2CH2O

Photosynthesis

ATP

Figure 8.12

54


Cristae O2 H2O

ADP ATP

NADH+H+

e–

e–

NADH+H+

e–

e–

e–

e–

Preparatory reaction Citric acidcycle

NADH+H+

and FADH2

Electron transport

chain

2 ATP2 ADP

4 ADP 4 ATP total

2 ATP net gain 2 ADP 2 32 ADP 32

or 34 or 34

ATP ATP

MatrixNAD+

CH2O CO2

Cellular Respiration

Glycolysis

glucose pyruvate

NADH

Figure 8.12

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Thylakoidmembrane H2O O2 Cristae O2 H2O

ADP ATPADP ATP

solar

energy

H2O CO2

NADH+H+

e–

e–

NADH+H+e–

e–

e–

e–

Preparatory reaction Citric acid

cycle

NADH + H+

and FADH2

Electron transportchain

2 ATP2 ADP

4 ADP 4 ATP total

2 ATP net gain 2 ADP 2 32 ADP 32or 34 or 34

ATP ATP

MatrixNAD+

CH2O CO2

Cellular Respiration

Lightreactions

ADP + P

Calvincycle

reactions

NADP+

NADPH

thylakoid

membrane O2 CH2O

stroma

StromaNADPH NADP+

CO2 CH2O

Photosynthesis

Glycolysis

glucose pyruvate

ATP

NADH

Figure 8.12

07-chapt08 lecture anim - nassau community college · 8.1 overview of photosynthesis ... 8.2 plants...

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