Biology 201 Chapter 8

Chapter 08

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Photosynthesis

Chapter 8

2

3

Photosynthesis Overview

• Energy for all life on Earth ultimately comes from photosynthesis

6CO2 + 12H2O C6H12O6 + 6H2O + 6O2

• Oxygenic photosynthesis is carried out by– Cyanobacteria– 7 groups of algae– All land plants – chloroplasts

Chloroplast

• Thylakoid membrane – internal membrane– Contains chlorophyll and other photosynthetic

pigments– Pigments clustered into photosystems

• Grana – stacks of flattened sacs of thylakoid membrane

• Stroma lamella – connect grana• Stroma – semiliquid surrounding thylakoid

membranes4

5


Vascular bundle Stoma

CuticleEpidermis

Mesophyll

ChloroplastInner membraneOuter membrane

Cell wall

1.58 mm

Vacuole

Courtesy Dr. Kenneth Miller, Brown University

6

Stages

• Light-dependent reactions– Require light1.Capture energy from sunlight2.Make ATP and reduce NADP+ to NADPH

• Carbon fixation reactions or light-independent reactions– Does not require light3.Use ATP and NADPH to synthesize organic

molecules from CO2

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O2

Stroma

Photosystem

Thylakoid

NADP+ADP + Pi

CO2

Sunlight

Photosystem

Light-DependentReactions

CalvinCycle

Organicmolecules

O2

ATP NADPH

H2O


8

Discovery of Photosynthesis

• Jan Baptista van Helmont (1580–1644)– Demonstrated that the substance of the plant

was not produced only from the soil• Joseph Priestly (1733–1804)

– Living vegetation adds something to the air• Jan Ingenhousz (1730–1799)

– Proposed plants carry out a process that uses sunlight to split carbon dioxide into carbon and oxygen (O2 gas)

• F.F. Blackman (1866–1947)– Came to the startling

conclusion that photosynthesis is in fact a multistage process, only one portion of which uses light directly

– Light versus dark reactions

– Enzymes involved

9

Maximum rate

Temperature limitedExcess CO2; 20ºC

CO2 limited

Light Intensity (foot-candles)500 1000 1500 2000 2500

Incr

ease

d R

ate

of P

hoto

synt

hesi

s

0

Excess CO2; 35ºC

Insufficient CO2 (0.01%); 20ºC

Ligh

t lim

ited


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• C. B. van Niel (1897–1985)– Found purple sulfur bacteria do not release O2

but accumulate sulfur– Proposed general formula for photosynthesis

• CO2 + 2 H2A + light energy → (CH2O) + H2O + 2 A

– Later researchers found O2 produced comes from water

• Robin Hill (1899–1991)– Demonstrated Niel was right that light energy

could be harvested and used in a reduction reaction

11

Pigments

• Molecules that absorb light energy in the visible range

• Light is a form of energy• Photon – particle of light

– Acts as a discrete bundle of energy– Energy content of a photon is inversely

proportional to the wavelength of the light• Photoelectric effect – removal of an electron

from a molecule by light

12


400 nm

Visible light

430 nm 500 nm 560 nm 600 nm 650 nm 740 nm

1 nm0.001 nm 10 nm 1000 nmIncreasing wavelength

Increasing energy

0.01 cm 1 cm 1 m

Radio wavesInfraredX-raysGamma rays

100 m

UVlight

13

Absorption spectrum

• When a photon strikes a molecule, its energy is either – Lost as heat– Absorbed by the electrons of the molecule

• Boosts electrons into higher energy level

• Absorption spectrum – range and efficiency of photons molecule is capable of absorbing

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Wavelength (nm)400 450 500 550 600 650 700

Ligh

tA

bsor

btio

n

low

highcarotenoidschlorophyll achlorophyll b


• Organisms have evolved a variety of different pigments

• Only two general types are used in green plant photosynthesis– Chlorophylls– Carotenoids

• In some organisms, other molecules also absorb light energy

15

Pigments in Photosynthesis

Chlorophylls

• Chlorophyll a– Main pigment in plants and cyanobacteria– Only pigment that can act directly to convert

light energy to chemical energy– Absorbs violet-blue and red light

• Chlorophyll b– Accessory pigment or secondary pigment

absorbing light wavelengths that chlorophyll a does not absorb

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• Structure of chlorophyll

• porphyrin ring– Complex ring structure

with alternating double and single bonds

– Magnesium ion at the center of the ring

• Photons excite electrons in the ring

• Electrons are shuttled away from the ring

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H2C CH

CH2CH3

H

H

H

CO

CHCCH3

CHCH3

CH2

CH2

CH2

CHCH3

CH2

CH2

CH2

CHCH3

CH3

O

CO2CH3O

N N

N N

Mg

H

HChlorophyll a: = CH3

Chlorophyll b: = CHO

R

RR

HPorphyrinhead

H3C

H3CCH3

CH2

CH2

CH2

CH2CH2

CH2

Hydrocarbontail


• Action spectrum– Relative effectiveness of different

wavelengths of light in promoting photosynthesis

– Corresponds to the absorption spectrum for chlorophylls

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Ligh

tA

bsor

btio

n

low

high Oxygen-seeking bacteria

Filament of green algae


• Carotenoids– Carbon rings linked to

chains with alternating single and double bonds

– Can absorb photons with a wide range of energies

– Also scavenge free radicals – antioxidant

• Protective role

• Phycobiloproteins– Important in low-light

ocean areas

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Oak leafin summer

Oak leafin autumn

© Eric Soder/pixsource.com

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Photosystem Organization

• Antenna complex– Hundreds of accessory pigment molecules– Gather photons and feed the captured light

energy to the reaction center• Reaction center

– 1 or more chlorophyll a molecules– Passes excited electrons out of the

photosystem

Antenna complex

• Also called light-harvesting complex• Captures photons from sunlight and

channels them to the reaction center chlorophylls

• In chloroplasts, light-harvesting complexes consist of a web of chlorophyll molecules linked together and held tightly in the thylakoid membrane by a matrix of proteins

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22


e–Photon

Photosystem

Thylakoid membrane

Chlorophyllmolecule

Electronacceptor

Reaction centerchlorophyll

Thylakoid membrane

Electrondonor e–

Reaction center

• Transmembrane protein–pigment complex• When a chlorophyll in the reaction center

absorbs a photon of light, an electron is excited to a higher energy level

• Light-energized electron can be transferred to the primary electron acceptor, reducing it

• Oxidized chlorophyll then fills its electron “hole” by oxidizing a donor molecule

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Light

e–

–+–+

Excitedchlorophyllmolecule

Electrondonor

Electronacceptor

Chlorophyllreduced

Chlorophylloxidized

Donoroxidized Acceptor

reduced

e–

e– e–

e–

e–

e–

e–

25

Light-Dependent Reactions1. Primary photoevent

– Photon of light is captured by a pigment molecule2. Charge separation

– Energy is transferred to the reaction center; an excited electron is transferred to an acceptor molecule

3. Electron transport– Electrons move through carriers to reduce NADP+

4. Chemiosmosis– Produces ATP

Cap

ture

of l

ight

ene

rgy

26

• In sulfur bacteria, only one photosystem is used

• Generates ATP via electron transport• Anoxygenic photosynthesis• Excited electron passed to electron

transport chain• Generates a proton gradient for ATP

synthesis

Cyclic photophosphorylation

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Ener

gy o

f ele

ctro

ns

High

Low

e–

Photon

Photosystem

Excited reaction center

Electronacceptor

Electronacceptor

Reactioncenter (P870)

b-c1

complex ATPe–

e–

28

Chloroplasts have two connected photosystems

• Oxygenic photosynthesis• Photosystem I (P700)

– Functions like sulfur bacteria

• Photosystem II (P680)– Can generate an oxidation potential high enough to

oxidize water• Working together, the two photosystems carry out

a noncyclic transfer of electrons that is used to generate both ATP and NADPH

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• Photosystem I transfers electrons ultimately to NADP+, producing NADPH

• Electrons lost from photosystem I are replaced by electrons from photosystem II

• Photosystem II oxidizes water to replace the electrons transferred to photosystem I

• 2 photosystems connected by cytochrome/ b6-f complex

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Noncyclic photophosphorylation

• Plants use photosystems II and I in series to produce both ATP and NADPH

• Path of electrons not a circle• Photosystems replenished with electrons

obtained by splitting water• Z diagram

31


Ener

gy o

f ele

ctro

ns

Photon



Plastoquinone

Plastocyanin

Ferredoxin

Photosystem II

Photosystem I

Photon

b6-fcomplex

3. A pair of chlorophylls in the reaction center absorb two photons. This

excites two electrons that are passed to NADP+, reducing it to NADPH. Electron transport from photosystem II replaces these electrons.

H2O

H+PC

Fd

2H+ + 1/2O2

NADP+ + H+

2

2

2

2

2

1. A pair of chlorophylls in the reaction center absorb two photons of light. This excites two electrons that are transferred to plastoquinone (PQ). Loss of electrons from the reaction center produces an oxidation potential capable of oxidizing water.

Reactioncenter

Proton gradient formedfor ATP synthesis

Reactioncenter

e–

e–

PQ

e–

NADPreductase

NADPHe–

2. The electrons pass through the b6-f complex, which uses the energy released to pump protons across the thylakoid membrane. The proton gradient is used to produce ATP by chemiosmosis.

e–

Two photosystems

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Rat

e of

Pho

tosy

nthe

sis

low

high

Far-red light on Both lights onRed light onOff OffTime

Off

33

Photosystem II

• Resembles the reaction center of purple bacteria• Core of 10 transmembrane protein subunits with

electron transfer components and two P680 chlorophyll molecules

• Reaction center differs from purple bacteria in that it also contains four manganese atoms– Essential for the oxidation of water

• b6-f complex– Proton pump embedded in thylakoid membrane

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Photosystem I

• Reaction center consists of a core transmembrane complex consisting of 12 to 14 protein subunits with two bound P700 chlorophyll molecules

• Photosystem I accepts an electron from plastocyanin into the “hole” created by the exit of a light-energized electron

• Passes electrons to NADP+ to form NADPH

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Photosystem II Photosystem Ib6-f complex

Stroma

PlastoquinoneProton

gradientPlastocyanin Ferredoxin

H+

H+H+

H+

NADPH

ATPADP

+ NADP+

NADPHNADPATPADP + Pi

CalvinCycle

Photon Photon

H2O

e– e–

e–

Fd

PC

PQ

1. Photosystem II absorbs photons, exciting electrons that are passed to plastoquinone (PQ). Electrons lost from photosystem II are replaced by the oxidation of water, producing O2

2. The b6-f complex receives electrons from PQ and passes them to plastocyanin (PC). This provides energy for the b6-f complex to pump protons into the thylakoid.

3. Photosystem I absorbs photons, exciting electrons that are passed through a carrier to reduce NADP+ to NADPH. These electrons are replaced by electron transport from photosystem II.

4. ATP synthase uses the proton gradient to synthesize ATP from ADP and Pi

enzyme acts as a channel for protons to diffuse back into the stroma using this energy to drive the synthesis of ATP.

NADPreductase

ATPsynthase

1/2O2 2H+

Water-splittingenzyme

Thylakoidspace

AntennacomplexThylakoid

membrane


H+

H+

e–22 22

22

22

Chemiosmosis

• Electrochemical gradient can be used to synthesize ATP

• Chloroplast has ATP synthase enzymes in the thylakoid membrane– Allows protons back into stroma

• Stroma also contains enzymes that catalyze the reactions of carbon fixation – the Calvin cycle reactions

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Production of additional ATP

• Noncyclic photophosphorylation generates– NADPH– ATP

• Building organic molecules takes more energy than that alone

• Cyclic photophosphorylation used to produce additional ATP– Short-circuit photosystem I to make a larger

proton gradient to make more ATP37

38

Carbon Fixation – Calvin Cycle

• To build carbohydrates cells use• Energy

– ATP from light-dependent reactions– Cyclic and noncyclic photophosphorylation– Drives endergonic reaction

• Reduction potential– NADPH from photosystem I– Source of protons and energetic electrons

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Calvin cycle

• Named after Melvin Calvin (1911–1997)• Also called C3 photosynthesis

• Key step is attachment of CO2 to RuBP to form PGA

• Uses enzyme ribulose bisphosphate carboxylase/oxygenase or rubisco

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3 phases

1. Carbon fixation– RuBP + CO2 → PGA

2. Reduction– PGA is reduced to G3P

3. Regeneration of RuBP– PGA is used to regenerate RuBP

• 3 turns incorporate enough carbon to produce a new G3P

• 6 turns incorporate enough carbon for 1 glucose

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4 Pi

12 NADP+

12

12 ADP

NADPHNADP+ADP+ Pi


CalvinCycle

6 molecules of12 molecules of

12 molecules of

1,3-bisphosphoglycerate (3C)

12 molecules of

Glyceraldehyde 3-phosphate (3C) (G3P)

10 molecules of

Glyceraldehyde 3-phosphate (3C) (G3P)

Stroma of chloroplast6 molecules of

Carbondioxide (CO2)

12 ATP

6 ADP

6 ATP

Rubisco

Calvin Cycle

Pi

Ribulose 1,5-bisphosphate (5C) (RuBP)3-phosphoglycerate (3C) (PGA)

Glyceraldehyde 3-phosphate (3C)

2 molecules of

Glucose andother sugars

12 NADPH

ATP

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Output of Calvin cycle

• Glucose is not a direct product of the Calvin cycle

• G3P is a 3 carbon sugar– Used to form sucrose

• Major transport sugar in plants• Disaccharide made of fructose and glucose

– Used to make starch• Insoluble glucose polymer• Stored for later use

44

Energy cycle

• Photosynthesis uses the products of respiration as starting substrates

• Respiration uses the products of photosynthesis as starting substrates

• Production of glucose from G3P even uses part of the ancient glycolytic pathway, run in reverse

• Principal proteins involved in electron transport and ATP production in plants are evolutionarily related to those in mitochondria

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O2

Heat

ATP NADPH NADH

ATP

Sunlight

Pyruvate

CO2

Glucose

ADP + Pi NAD+NADP+

H2O

Photo-system

II

Photo-system

I

ElectronTransportSystem

ADP + Pi

ADP + Pi

ATP

ATP

CalvinCycle

KrebsCycle

46

Photorespiration

• Rubisco has 2 enzymatic activities– Carboxylation

• Addition of CO2 to RuBP• Favored under normal conditions

– Photorespiration• Oxidation of RuBP by the addition of O2

• Favored when stoma are closed in hot conditions• Creates low-CO2 and high-O2

• CO2 and O2 compete for the active site on RuBP

47


Heat

Stomata

O2O2

CO2 CO2

Under hot, arid conditions, leaves lose water byevaporation through openings in the leavescalled stomata.

The stomata close to conserve water but as aresult, O2 builds up inside the leaves, and CO2

cannot enter the leaves.

Leafepidermis

H2OH2O

48

Types of photosynthesis

• C3

– Plants that fix carbon using only C3 photosynthesis (the Calvin cycle)

• C4 and CAM– Add CO2 to PEP to form 4 carbon molecule– Use PEP carboxylase– Greater affinity for CO2, no oxidase activity

– C4 – spatial solution– CAM – temporal solution

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CO2

RuBP

3PG(C3)

a. C4 pathway

Bundle-sheath cellMesophyll cell

Stoma Vein

G3P

b. C4 pathwayStoma Vein

Mesophyll cell

G3P

CO2

CO2

C4

Bundle-sheath cell

Mesophyllcell

Bundle-sheathcell

CalvinCycle

Mesophyllcell

CalvinCycle

a: © John Shaw/Photo Researchers, Inc. b: © Joseph Nettis/National Audubon Society Collection/Photo Researchers, Inc.

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C4 plants

• Corn, sugarcane, sorghum, and a number of other grasses

• Initially fix carbon using PEP carboxylase in mesophyll cells

• Produces oxaloacetate, converted to malate, transported to bundle-sheath cells

• Within the bundle-sheath cells, malate is decarboxylated to produce pyruvate and CO2

• Carbon fixation then by rubisco and the Calvin cycle

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Oxaloacetate

Pyruvate Malate

Glucose

MalatePyruvate

+ Pi

Mesophyllcell

Phosphoenolpyruvate (PEP)

Bundle-sheathcell

CalvinCycle

AMP +PPi

ATP

CO2

CO2

• C4 pathway, although it overcomes the problems of photorespiration, does have a cost

• To produce a single glucose requires 12 additional ATP compared with the Calvin cycle alone

• C4 photosynthesis is advantageous in hot dry climates where photorespiration would remove more than half of the carbon fixed by the usual C3 pathway alone

52

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CAM plants

• Many succulent (water-storing) plants, such as cacti, pineapples, and some members of about two dozen other plant groups

• Stomata open during the night and close during the day– Reverse of that in most plants

• Fix CO2 using PEP carboxylase during the night and store in vacuole

• When stomata closed during the day, organic acids are decarboxylated to yield high levels of CO2

• High levels of CO2 drive the Calvin cycle and minimize photorespiration

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night

day

CO2

CO2

C4

G3P

CalvinCycle

(inset): © 2011 Jessica Solomatenko/Getty Images RF

Compare C4 and CAM

• Both use both C3 and C4 pathways

• C4 – two pathways occur in different cells

• CAM – C4 pathway at night and the C3 pathway during the day

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Education

Biology 201 Chapter 8