Chapter 10Chapter 10

Photosynthesis

Concept 10.1: Photosynthesis converts light energy to the chemical energy of food

• Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria

• The structural organization of these cells allows for the chemical reactions of photosynthesis

I. Chloroplasts: Photosynthesis in Plants

• Leaves = major locations of photo

• Chlorophyll = green pigment w/in chloroplasts, absorbs light energy to make organic molecules in chloroplast

• CO2 enters and O2 exits leaf through microscopic pores = stomata

• Found mainly in cells of the mesophyll (interior tissue of leaf)

• Typical mesophyll cell has 30–40 chloroplasts

• Chlorophyll is in membranes of thylakoids (connected sacs in the chloroplast); thylakoids may be stacked in columns called grana

• Stroma - dense fluid inside chloroplasts

Fig. 10-3a

5 µm

Mesophyll cell

StomataCO2 O2

Chloroplast

Mesophyll

Vein

Leaf cross section

Fig. 10-3b

1 µm

Thylakoidspace

Chloroplast

GranumIntermembranespace

Innermembrane

Outermembrane

Stroma

Thylakoid

II. Tracking Atoms Through Photosynthesis

• Photosynthesis summarized as:

6 CO2 + 12 H2O + Light energy C6H12O6 + 6 O2 + 6 H2O

A. The Splitting of Water

• Chloroplasts split H2O into H and O, incorporating the electrons of H into sugar molecules

Reactants: 6 CO2

Products:

12 H2O

6 O26 H2OC6H12O6

• Photosynthesis is a redox process in which H2O is oxidized and CO2 is reduced

III. Two Stages of Photosynthesis: A Preview

• Photosynthesis = light reactions (photo part) and Calvin cycle (synthesis part)

• Light reactions (in the thylakoids):

– Split H2O

– Release O2

– Reduce NADP+ to NADPH

– Generate ATP from ADP by photophosphorylation

• Calvin cycle (in the stroma) forms sugar from CO2, using ATP and NADPH. Begins with carbon fixation, incorporating CO2 into organic molecules

Light

Fig. 10-5-1

H2O

Chloroplast

LightReactions

NADP+

P

ADP

i+

Light

Fig. 10-5-2

H2O

Chloroplast

LightReactions

NADP+

P

ADP

i+

ATP

NADPH

O2

Light

Fig. 10-5-3

H2O

Chloroplast

LightReactions

NADP+

P

ADP

i+

ATP

NADPH

O2

CalvinCycle

CO2

Light

Fig. 10-5-4

H2O

Chloroplast

LightReactions

NADP+

P

ADP

i+

ATP

NADPH

O2

CalvinCycle

CO2

[CH2O]

(sugar)

Concept 10.2: The light reactions convert solar energy to the chemical energy of ATP and NADPH

• Chloroplasts are solar-powered chemical factories

• Thylakoids transform light energy into chemical energy of ATP and NADPH

IV. Nature of Sunlight

• Light = form of electromagnetic energy, also called electromagnetic radiation

• Light travels in waves

• Wavelength = dist between crests of waves

• Wavelength determines the type of electromagnetic energy

• Electromagnetic spectrum = range of electromagnetic energy (radiation)

• Visible light = wavelengths (inc. those for photosynthesis) that produce colors we can see

• Light also acts as though it consists of small particles (photons)

UV

Fig. 10-6

Visible light

InfraredMicro-waves

RadiowavesX-raysGamma

rays

103 m1 m

(109 nm)106 nm103 nm1 nm10–3 nm10–5 nm

380 450 500 550 600 650 700 750 nm

Longer wavelength

Lower energyHigher energy

Shorter wavelength

V. Photosynthetic Pigments

• Absorb visible light

• Different pigments absorb different wavelengths

• Wavelengths that are not absorbed are reflected or transmitted

• Leaves appear green b/c chlorophyll reflects and transmits green light

Fig. 10-7

Reflectedlight

Absorbedlight

Light

Chloroplast

Transmittedlight

Granum

Galvanometer

Slit moves topass lightof selectedwavelength

Whitelight

Greenlight

Bluelight

The low transmittance(high absorption)reading indicates thatchlorophyll absorbsmost blue light.

The high transmittance(low absorption)reading indicates thatchlorophyll absorbsvery little green light.

Refractingprism

Photoelectrictube

Chlorophyllsolution

1

2 3

4

• Spectrophotometer = measures a pigment’s ability to absorb various wavelengths

• Absorption spectrum = graph plotting a pigment’s light absorption vs wavelength

• Absorption spectrum of chlorophyll a suggests violet-blue and red light work best

• Action spectrum profiles the effectiveness of different wavelengths of radiation in a process

Fig. 10-9

Wavelength of light (nm)

(b) Action spectrum

(a) Absorption spectra

(c) Engelmann’s experiment

Aerobic bacteria

RESULTS

Ra

te o

f p

ho

tos

yn

the

sis

(me

as

ure

d b

y O

2 re

lea

se

)A

bs

orp

tio

n o

f li

gh

t b

yc

hlo

rop

las

t p

igm

en

ts

Filamentof alga

Chloro- phyll a Chlorophyll b

Carotenoids

500400 600 700

700600500400

• Chlorophyll a = main photosynthetic pigment

• Accessory pigments

– chlorophyll b = broaden spectrum used for photosynthesis

– Carotenoids = absorb excessive light that would damage chlorophyll

• Overview Photosynthesis Structures

Fig. 10-10

Porphyrin ring:light-absorbing“head” of molecule;note magnesiumatom at center

in chlorophyll aCH3

Hydrocarbon tail:interacts with hydrophobicregions of proteins insidethylakoid membranes ofchloroplasts; H atoms notshown

CHO in chlorophyll b

VI. Excitation of Chlorophyll by Light

• Pigment absorbs light, it goes from a ground state to an excited state (unstable)

• Excited e-s fall back to ground state, photons are given off, an afterglow called fluorescence

• If illuminated, an isolated solution of chlorophyll will fluoresce, giving off light and heat

Fig. 10-11

(a) Excitation of isolated chlorophyll molecule

Heat

Excitedstate

(b) Fluorescence

Photon Groundstate

Photon(fluorescence)

En

erg

y o

f el

ectr

on

e–

Chlorophyllmolecule

VII. Photosystem

• Photosystem = a reaction-center complex (protein) surrounded by light-harvesting complexes

• Light-harvesting complexes (pigment molecules bound to proteins) funnel energy of photons to reaction center

• Primary electron acceptor in reaction center accepts an excited e- from chlorophyll a

• This solar-powered transfer of an e- from chlorophyll a to PEA is first step of light reactions

Fig. 10-12

THYLAKOID SPACE(INTERIOR OF THYLAKOID)

STROMA

e–

Pigmentmolecules

Photon

Transferof energy

Special pair ofchlorophyll amolecules

Th

yla

koid

me

mb

ran

e

Photosystem

Primaryelectronacceptor

Reaction-centercomplex

Light-harvestingcomplexes

• Two types of photosystems in thylakoid membrane

– Photosystem II (PS II) functions first (# reflects order of discovery) and is best at absorbing a wavelength of 680 nm

• The reaction-center chlorophyll a of PS II is called P680

– Photosystem I (PS I) is best at absorbing a wavelength of 700 nm

• The reaction-center chlorophyll a of PS I is called P700

• Photosynthesis Part 1 Bleier (15 min)

VIII. Linear Electron Flow

• During light reactions, there are two possible routes for e- flow: cyclic and linear

• Linear electron flow (primary pathway) involves both photosystems and produces ATP and NADPH using light energy

• Photon hits pigment and its energy is passed among pigment molecules until it excites P680

• An excited e- from P680 is transferred to the primary electron acceptor

Pigmentmolecules

Light

P680

2

1

Photosystem II(PS II)

Primaryacceptor

• P680+ (P680 missing an e-) is a very strong oxidizing agent

• H2O is split by enzymes, and e-s are transferred from H atoms to P680+, reducing it to P680

• O2 is released as a by-product

Pigmentmolecules

Light

P680

e–

Primaryacceptor

2

1

e–

e–

2 H+

O2

+3

H2O

1/2

Fig. 10-13-2

Photosystem II(PS II)

• Each e- “falls” down an electron transport chain from primary electron acceptor of PS II to PS I

• Energy released by the fall drives the creation of a proton gradient across the thylakoid membrane

• Diffusion of H+ (protons) across the membrane drives ATP synthesis (sound familiar???)

Pigmentmolecules

Light

P680

e–

Primaryacceptor

2

1

e–

e–

2 H+

O2

+3

H2O

1/2

4

Pq

Pc

Cytochromecomplex

Electron transport chain

5

ATP

Fig. 10-13-3

Photosystem II(PS II)

• PS I (like PS II), transferred light energy excites P700, which loses an e- to an electron acceptor

• P700+ (P700 missing an e-) accepts an electron passed down from PS II via the electron transport chain

Pigmentmolecules

Light

P680

e–

Primaryacceptor

2

1

e–

e–

2 H+

O2

+3

H2O

1/2

4

Pq

Pc

Cytochromecomplex

Electron transport chain

5

ATP

Photosystem I(PS I)

Light

Primaryacceptor

e–

P700

6

Fig. 10-13-4

Photosystem II(PS II)

• Each e- “falls” down an electron transport chain from the primary electron acceptor of PS I to the protein ferredoxin (Fd)

• e-s are then transferred to NADP+ and reduce it to NADPH

• e-s of NADPH are available for the Calvin cycle

• Photo ETC and ATP Production

Pigmentmolecules

Light

P680

e–

Primaryacceptor

2

1

e–

e–

2 H+

O2

+3

H2O

1/2

4

Pq

Pc

Cytochromecomplex

Electron transport chain

5

ATP

Photosystem I(PS I)

Light

Primaryacceptor

e–

P700

6

Fd

Electron transport chain

NADP+

reductase

NADP+

+ H+

NADPH

8

7

e–

e–

6

Fig. 10-13-5

Photosystem II(PS II)

Fig. 10-14

MillmakesATP

e–

NADPH

Ph

oto

n

e–

e–

e–

e–

e–

Ph

oto

n

ATP

Photosystem II Photosystem I

e–

IX. Cyclic Electron Flow

• Cyclic electron flow uses only photosystem I and produces ATP, but not NADPH

• Generates surplus ATP, satisfying the higher demand in the Calvin cycle

Fig. 10-15

ATPPhotosystem II

Photosystem I

Primary acceptor

Pq

Cytochromecomplex

Fd

Pc

Primaryacceptor

Fd

NADP+

reductaseNADPH

NADP+

+ H+

• Some organisms (purple sulfur bacteria) have PS I but not PS II

• Cyclic electron flow is thought to have evolved before linear electron flow and may protect cells from light-induced damage

• Cyclic vs Linear E- Flow

X. Comparison of Chloroplasts and Mitochondria

• Both generate ATP by chemiosmosis, but use different sources of energy

• Mitochondria transfer chemical energy from food to ATP; chloroplasts transform light energy into chemical energy of ATP

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

• In mitochondria, protons are pumped OUT to intermembrane space and drive ATP synthesis as they diffuse back INTO mitochondrial matrix

• In chloroplasts, protons are pumped INTO thylakoid space and drive ATP synthesis as they diffuse back OUT to the stroma

Fig. 10-16

Key

Mitochondrion Chloroplast

CHLOROPLASTSTRUCTURE

MITOCHONDRIONSTRUCTURE

Intermembranespace

Innermembrane

Electrontransport

chain

H+ Diffusion

Matrix

Higher [H+]Lower [H+]

Stroma

ATPsynthase

ADP + P i

H+ATP

Thylakoidspace

Thylakoidmembrane

• ATP and NADPH are produced on the side facing the stroma, where Calvin cycle takes place

• In summary, light reactions generate ATP and increase the potential energy of e-s by moving them from H2O to NADPH

Fig. 10-17

Light

Fd

Cytochromecomplex

ADP +

i H+

ATPP

ATPsynthase

ToCalvinCycle

STROMA(low H+ concentration)

Thylakoidmembrane

THYLAKOID SPACE(high H+ concentration)

STROMA(low H+ concentration)

Photosystem II Photosystem I

4 H+

4 H+

Pq

Pc

LightNADP+

reductase

NADP+ + H+

NADPH

+2 H+

H2OO2

e–

e–

1/21

2

3

XI. Calvin Cycle

• The Calvin cycle, like citric acid cycle, regenerates its starting material after molecules enter and leave the cycle

• Builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH

• Calvin Cycle

• Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde-3-phospate (G3P)

• For net synthesis of 1 G3P, the cycle must take place three times, fixing 3 molecules of CO2

• The Calvin cycle has three phases:

– Carbon fixation (catalyzed by rubisco)

– Reduction

– Regeneration of the CO2 acceptor (RuBP)

Fig. 10-18-1

Ribulose bisphosphate(RuBP)

3-Phosphoglycerate

Short-livedintermediate

Phase 1: Carbon fixation

(Entering oneat a time)

Rubisco

Input

CO2

P

3 6

3

3

P

PPP

Fig. 10-18-2

Ribulose bisphosphate(RuBP)

3-Phosphoglycerate

Short-livedintermediate

Phase 1: Carbon fixation

(Entering oneat a time)

Rubisco

Input

CO2

P

3 6

3

3

P

PPP

ATP6

6 ADP

P P6

1,3-Bisphosphoglycerate

6

P

P6

66 NADP+

NADPH

i

Phase 2:Reduction

Glyceraldehyde-3-phosphate(G3P)

1 POutput G3P

(a sugar)

Glucose andother organiccompounds

CalvinCycle

Fig. 10-18-3

Ribulose bisphosphate(RuBP)

3-Phosphoglycerate

Short-livedintermediate

Phase 1: Carbon fixation

(Entering oneat a time)

Rubisco

Input

CO2

P

3 6

3

3

P

PPP

ATP6

6 ADP

P P6

1,3-Bisphosphoglycerate

6

P

P6

66 NADP+

NADPH

i

Phase 2:Reduction

Glyceraldehyde-3-phosphate(G3P)

1 POutput G3P

(a sugar)

Glucose andother organiccompounds

CalvinCycle

3

3 ADP

ATP

5 P

Phase 3:Regeneration ofthe CO2 acceptor(RuBP)

G3P

• Photosynthesis Part 2 Bleier (15 min)

XII. Alternative carbon fixation

• Dehydration is a problem for plants

• Hot, dry days, plants close stomata, which conserves H2O but limits photosynthesis

– This reduces access to CO2 and causes O2 to build up

– Favor a wasteful process called photorespiration

XIII. Photorespiration

• In most plants (C3 plants), initial fixation of CO2, via rubisco, forms a three-carbon compound (thus the C3)

• In photorespiration, rubisco adds O2 instead of CO2 in the Calvin cycle

• Photoresp consumes O2 and organic fuel and releases CO2 w/o producing ATP or sugar

• May be an evolutionary relic because rubisco first evolved at a time when atmosphere had far less O2 and more CO2

• Limits damaging products of light reactions that build up in the absence of the Calvin cycle

• Photorespiration is a problem because on a hot, dry day it can drain as much as 50% of the carbon fixed by the Calvin cycle

XIV. C4 Plants

• C4 plants minimize the cost of photorespiration by incorporating CO2 into four-carbon compounds in mesophyll cells

• Requires enzyme PEP carboxylase

• PEP carboxylase has a higher affinity for CO2 than rubisco does; it can fix CO2 even when CO2 concentrations are low

• These four-carbon compounds are exported to bundle-sheath cells, where they release CO2 that is then used in the Calvin cycle

Fig. 10-19

C4 leaf anatomy

Mesophyll cellPhotosyntheticcells of C4

plant leafBundle-sheathcell

Vein(vascular tissue)

Stoma

The C4 pathway

Mesophyllcell CO2PEP carboxylase

Oxaloacetate (4C)

Malate (4C)

PEP (3C)ADP

ATP

Pyruvate (3C)

CO2

Bundle-sheathcell

CalvinCycle

Sugar

Vasculartissue

CAM Plants

• Some plants, including succulents, use crassulacean acid metabolism (CAM) to fix carbon

• CAM plants open their stomata at night, incorporating CO2 into organic acids

• Stomata close during the day, and CO2 is released from organic acids and used in the Calvin cycle

Fig. 10-20

CO2

Sugarcane

Mesophyllcell

CO2

C4

Bundle-sheathcell

Organic acidsrelease CO2 to Calvin cycle

CO2 incorporatedinto four-carbonorganic acids(carbon fixation)

Pineapple

Night

Day

CAM

SugarSugar

CalvinCycle

CalvinCycle

Organic acid Organic acid

(a) Spatial separation of steps (b) Temporal separation of steps

CO2 CO2

1

2

XV. Photosynthesis: A Review

• Sunlight gets stored as chemical energy in organic compounds

• Sugar supplies chemical energy and carbon skeletons to synthesize molecules of cells

• Plants store excess sugar as starch in roots, tubers, seeds, and fruits

• Produces the O2 in our atmosphere

• Overview - Anderson (13 min)

Fig. 10-21

LightReactions:

Photosystem II Electron transport chain

Photosystem I Electron transport chain

CO2

NADP+

ADP

P i+

RuBP 3-Phosphoglycerate

CalvinCycle

G3PATP

NADPHStarch(storage)

Sucrose (export)

Chloroplast

Light

H2O

O2

Fig. 10-UN1

CO2

NADP+

reductase

Photosystem II

H2O

O2

ATP

Pc

Cytochromecomplex

Primaryacceptor

Primaryacceptor

Photosystem I

NADP+

+ H+

Fd

NADPH

Electron transport

chain

Electron transport

chain

O2

H2O Pq

Fig. 10-UN2

Regeneration ofCO2 acceptor

1 G3P (3C)

Reduction

Carbon fixation

3 CO2

CalvinCycle

6 3C

5 3C

3 5C