Chapter 10: Photosynthesis

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Chapter 10—Photosynthesis

• Photosynthesis is the process that converts solar energy into chemical energy

• Directly or indirectly, photosynthesis nourishes almost the entire living world


• Autotrophs sustain themselves without eating anything derived from other organisms

• Autotrophs are the producers of the biosphere, producing organic molecules from CO2 and other inorganic molecules

• Almost all plants are photoautotrophs, using the energy of sunlight to make organic molecules from water and carbon dioxide


Figure 10.2

(a) Plants

(b) Multicellular alga

(c) Unicellular eukaryotes

(d) Cyanobacteria

(e) Purple sulfurbacteria

40μm

1 μm

10 μ

m

•Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotes•These organisms feed not only themselves but also the entire living world


• Heterotrophs obtain their organic material from other organisms

• Heterotrophs are the consumers of the biosphere

• Almost all heterotrophs, including humans, depend on photoautotrophs for food and oxygen


• Chloroplasts are organelles that are responsible for feeding the vast majority of organisms

• Chloroplasts are present in a variety of photosynthesizing organisms

A chloroplast has an envelope of two membranes surrounding a dense fluid called the stromaThylakoids are connected sacs in the chloroplast which compose a third membrane system

http://botit.botany.wisc.edu/images/130/Plant_Cell/Electron_Micrographs/Chloroplast_grana_RE.php


Chloroplasts: The Sites of Photosynthesis in Plants

• Leaves are the major locations of photosynthesis

• Their green color is from chlorophyll, the green pigment within chloroplasts

• Light energy absorbed by chlorophyll drives the synthesis of organic molecules in the chloroplast

• Through microscopic pores called stomata, CO2 enters the leaf and O2 exits


Stomate


• Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf

• A typical mesophyll cell has 30-40 chloroplasts

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

• Chloroplasts also contain stroma, a dense fluid


Figure 10.4

Stroma Granum

ThylakoidThylakoid

space

Outermembrane

Intermembranespace

Innermembrane

20 μm

Stomata

Chloroplast Mesophyllcell

1 μm

Mesophyll

Chloroplasts VeinLeaf cross section

CO2 O2


Tracking Atoms Through Photosynthesis: Scientific Inquiry

• Photosynthesis is a complex series of reactions that can be summarized as the following equation:6 CO2 + 12 H2O + Light energy C6H12O6 + 6 O2 + 6 H2 O

• Chloroplasts split water into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules


Figure 10.5

Reactants:

Products:

6 CO2 12 H2O

C6H12O6 6 H2O 6 O2


The Two Stages of Photosynthesis: A Preview

• Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part)

• The light reactions (in the thylakoids) split water, release O2, produce ATP, and form NADPH (photophosphorylation)

• The Calvin cycle (in the stroma) forms sugar from CO2, using ATP and NADPH

• The Calvin cycle begins with carbon fixation, incorporating CO2 into organic molecules


Figure 10.6-1Light

Thylakoid Stroma

Chloroplast

LIGHTREACTIONS

NADP+

ADP

P i

H2O


Figure 10.6-2Light

Thylakoid Stroma

Chloroplast

LIGHTREACTIONS

NADP+

ADP

P i

H2O

NADPH

ATP

O2


Figure 10.6-3Light

Thylakoid Stroma

Chloroplast

LIGHTREACTIONS

NADP+

ADP

P i

H2O

O2

CO2

NADPH

ATP

CALVINCYCLE


Figure 10.6-4Light

Thylakoid Stroma

Chloroplast

LIGHTREACTIONS

NADP+

ADP

P i

H2O

[CH2O](sugar)

CALVINCYCLE

CO2

NADPH

ATP

O2


The Nature of Sunlight

• Light is a form of electromagnetic energy, also called electromagnetic radiation

• Like other electromagnetic energy, light travels in rhythmic waves

• Wavelength = distance between crests of waves

• Wavelength determines the type of electromagnetic energy

• Light also behaves as though it consists of discrete particles, called photons


• The electromagnetic spectrum is the entire range of electromagnetic energy, or radiation

• Visible light consists of colors we can see, including wavelengths that drive photosynthesis


Figure 10.7

Visible light

Gammarays X-rays UV Infrared Micro-

wavesRadiowaves

380 450 500 550 600 650 700 750 nmShorter wavelength

Higher energy Lower energyLonger wavelength

10−5 10−3nm nm 1 nm 3 nm10 6 nm10 9(10nm)1 m

103 m


Photosynthetic Pigments: The Light Receptors

• Pigments are substances that absorb visible light

• Different pigments absorb different wavelengths

• Wavelengths that are not absorbed are reflected or transmitted

• Leaves appear green because chlorophyll reflects and transmits green light


Figure 10.8

Light

Chloroplast

Reflectedlight

Granum

Transmittedlight

Absorbedlight


Figure 10.9Whitelight

Refractingprism

Chlorophyllsolution

Photoelectrictube

Galvanometer

The high transmittance (lowabsorption) reading indicates that

chlorophyll absorbs verylittle green light.

Slit moves to pass lightof selected wavelength.

Greenlight

Bluelight

The low transmittance (highabsorption) reading indicates

that chlorophyll absorbsmost blue light.

12 3

4


• An absorption spectrum is a graph plotting a pigment’s light absorption versus wavelength

• The absorption spectrum of chlorophyll a suggests that violet-blue and red light work best for photosynthesis

• An action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving a process


PowerPoint Lectures for Biology, Seventh Edition

Neil Campbell and Jane Reece

Lectures by Chris Romero

LE 10-9a

Chlorophyll a

Chlorophyll b

Carotenoids

Wavelength of light (nm)

Absorption spectra

Abs

orpt

ion

of li

ght b

ych

loro

plas

t pig

men

ts

400 500 600 700


• The action spectrum of photosynthesis was first demonstrated in 1883 by Thomas Engelmann

• In his experiment, he exposed different segments of a filamentous alga to different wavelengths

• Areas receiving wavelengths favorable to photosynthesis produced excess O2

• He used aerobic bacteria clustered along the alga as a measure of O2 production


Figure 10.10

Chloro-phyll a Chlorophyll b

Carotenoids

400 500 600 700Wavelength of light (nm)

(a) Absorption spectraA

bsor

ptio

nof

ligh

t by

chlo

ropl

ast

pigm

ents

Rat

e of

phot

osyn

thes

is(m

easu

red

by O

2re

leas

e)

400 500 600 700

400 500 600 700

(b) Action spectrum

(c) Engelmann’s experiment

Aerobic bacteriaFilament of alga


• Chlorophyll a is the main photosynthetic pigment

• Accessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesis

• Accessory pigments called carotenoids absorb excessive light that would damage chlorophyll


Figure 10.11

Porphyrin ring:light-absorbing

“head” of molecule;note magnesium

atom at center

Hydrocarbon tail:interacts with hydrophobicregions of proteins inside

thylakoid membranes ofchloroplasts; H atoms not

shown

CH in chlorophyll ain chlorophyll b

3CHOCH3


Excitation of Chlorophyll by Light

• When a pigment absorbs light, it goes from a ground state to an excited state, which is unstable

• When excited electrons fall back to the ground state, photons are given off, an afterglow called fluorescence

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


Figure 10.12

Excitedstate

Heat

(a) Excitation of isolated chlorophyll molecule (b) Fluorescence

Groundstate

Photon(fluorescence)

PhotonChlorophyll

molecule

Ener

gy o

f ele

ctro

n

e−


A Photosystem: A Reaction Center Associated with Light-Harvesting Complexes

• A photosystem consists of a reaction center surrounded by light-harvesting complexes

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

• A primary electron acceptor in the reaction center accepts an excited electron from chlorophyll a

• Solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions


Figure 10.13

(a) How a photosystem harvests light (b) Structure of a photosystem

Chlorophyll STROMA

THYLA-KOID

SPACEProtein

subunits

Thyl

akoi

d m

embr

ane

Pigmentmolecules

Primaryelectronacceptor

Reaction-center

complex

STROMA

Photosystem

Light-harvestingcomplexes

Photon

Transferof energy

Special pair of chloro-phyll a molecules

THYLAKOID SPACE(INTERIOR OF THYLAKOID)

Thyl

akoi

d m

embr

ane

e−


• There are two types of photosystems in the thylakoid membrane

• Photosystem II functions first (the numbers reflect order of discovery) and is best at absorbing a wavelength of 680 nm

• Photosystem I is best at absorbing a wavelength of 700 nm

• The two photosystems work together to use light energy to generate ATP and NADPH


Noncyclic Electron Flow

• During the light reactions, there are two possible routes for electron flow: cyclic and noncyclic

• Noncyclic electron flow, the primary pathway, involves both photosystems and produces ATP and NADPH

• There are 8 steps in linear electron flow:

1. A photon hits a pigment and its energy is passed among pigment molecules until it excites P680

2. An excited electron from P680 is transferred to the primary electron acceptor (we now call it P680)


Figure 10.14-1

Pigmentmolecules

e−

1

2

P680Light

Photosystem II(PS II)

Primaryacceptor


Figure 10.14-2

Pigmentmolecules

e−

1

2

P680Light


Primaryacceptor

3e−e−

2 H

O2

H2O

½

3. H2O is split by enzymes, and the electrons are transferred from the hydrogen atoms to P680, thus reducing it to P680

– P680 is the strongest known biological oxidizing agent

– O2 is released as a by-product of this reaction


Figure 10.14-3

Pigmentmolecules

e−

1

2

P680Light


Primaryacceptor

3e−e−

2 H

O2

H2O

ATP

4

5

Electrontransport

chain

Cytochromecomplex

Pq

Pc½

4. Each electron “falls” down an electron transport chain from the primary electron acceptor of PS II to PS I

5. 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


Figure 10.14-4

Pigmentmolecules

e−

1

2

P680Light


Primaryacceptor

3e−e−

2 H

O2

H2O

ATP

4

5

Electrontransport

chain

Cytochromecomplex

Pq

PcP700

Light

Photosystem I(PS I)

6

Primaryacceptor

e−

½

6. In PS I (like PS II), transferred light energy excites P700, which loses an electron to an electron acceptor

– P700 (P700 that is missing an electron) accepts an electron passed down from PS II via the electron transport chain


Figure 10.14-5

Pigmentmolecules

e−

1

2

P680Light


Primaryacceptor

3e−e−

2 H

O2

H2O

ATP

4

5

Electrontransport

chain

Cytochromecomplex

Pq

PcP700

Light

Photosystem I(PS I)

6

Primaryacceptor

e−

e−

7

8Fd

e−

Electrontransport

chain

NADP

reductaseNADPH

NADP

H

½


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

8. The electrons are then transferred to NADP and reduce it to NADPH

– The electrons of NADPH are available for the reactions of the Calvin cycle

– This process also removes an H from the stroma


Cyclic Electron Flow

• Cyclic electron flow uses only photosystem I and produces only ATP

• Cyclic electron flow generates surplus ATP, satisfying the higher demand in the Calvin cycle


Figure 10.16

Primaryacceptor

Primaryacceptor

Fd

Cytochromecomplex

Pc

Pq

Photosystem IIPhotosystem I

FdNADP

HNADP

reductase NADPH

ATP


A Comparison of Chemiosmosis in Chloroplasts and Mitochondria

• Chloroplasts and mitochondria generate ATP by chemiosmosis, but use different sources of energy

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

• The spatial organization of chemiosmosis differs in chloroplasts and mitochondria


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

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


Figure 10.17

MITOCHONDRIONSTRUCTURE

CHLOROPLASTSTRUCTURE

Thylakoidmembrane

Stroma

ATP

Thylakoidspace

Inter-membrane

space

Innermembrane

MatrixKey

Diffusion

Electrontransport

chain

ATPsynthase

ADP H

H

Higher [H]Lower [H]

P i


View the video reviewing photosynthesis


4. Which makes an INCORRECT comparison between the membrane and surrounding compartments indicated in mitochondria and chloroplasts by the boxes (see figure)?

a. The darker compartment will often be more positively charged and more acidic.

b. The flow of electrons between items in the membrane results in protons being pumped from the darker to the lighter compartments.

c. The lighter compartment is where much of the carbon metabolism is done.

d. This membrane has an ATP synthase in it.

e. The lighter compartments are both similar to the cytosolic compartment of bacteria.


4. Which makes an INCORRECT comparison between the membrane and surrounding compartments indicated in mitochondria and chloroplasts by the boxes (see figure)?

a. The darker compartment will often be more positively charged and more acidic.

b. The flow of electrons between items in the membrane results in protons being pumped from the darker to the lighter compartments. (not true, pumped into darker compartments)

c. The lighter compartment is where much of the carbon metabolism is done.

d. This membrane has an ATP synthase in it.

e. The lighter compartments are both similar to the cytosolic compartment of bacteria.


• The current model for the thylakoid membrane is based on studies in several laboratories

• Water is split by photosystem II on the side of the membrane facing the thylakoid space

• The diffusion of H+ from the thylakoid space back to the stroma powers ATP synthase

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


Figure 10.18

Photosystem II Photosystem ICytochromecomplex

Light

Pq

Light 4 H

2 H 4 HO2

H2OPc

Fd3

21

NADP

ToCalvinCycle

NADP

reductase

STROMA(low H concentration)

ATPsynthase

THYLAKOID SPACE(high H concentration)

Thylakoidmembrane

ADP

HATP

P i

e e

NADPH

½

H


Concept 10.3: The Calvin cycle uses ATP and NADPH to convert CO2 to sugar

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

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

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

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


• The Calvin cycle has three phases:

– Carbon fixation (catalyzed by rubisco)

– Reduction

– Regeneration of the CO2 acceptor (RuBP)





LE 10-18_1

[CH2O] (sugar)O2

NADPH

ATP

ADPNADP+

CO2H2O

LIGHTREACTIONS

CALVINCYCLE

LightInput

3CO2

(Entering oneat a time)

Rubisco

3 P PShort-lived

intermediate

Phase 1: Carbon fixation

6 P3-Phosphoglycerate

6 ATP

6 ADP

CALVINCYCLE

3 P PRibulose bisphosphate

(RuBP)





LE 10-18_2

[CH2O] (sugar)O2

NADPH

ATP

ADPNADP+

CO2H2O

LIGHTREACTIONS

CALVINCYCLE

Light Input

CO2


Rubisco

3 P PShort-lived

intermediate



6 ATP

6 ADP

CALVINCYCLE

3

P PRibulose bisphosphate

(RuBP)

3

6 NADP+

6

6 NADPH

P i

6 P1,3-Bisphosphoglycerate

P

6 PGlyceraldehyde-3-phosphate

(G3P)

P1G3P

(a sugar)Output

Phase 2:Reduction

Glucose andother organiccompounds





LE 10-18_3

[CH2O] (sugar)O2

NADPH

ATP

ADPNADP+

CO2H2O

LIGHTREACTIONS

CALVINCYCLE

Light Input

CO2


Rubisco

3 P PShort-lived

intermediate



6 ATP

6 ADP

CALVINCYCLE

3

P PRibulose bisphosphate

(RuBP)

3

6 NADP+

6

6 NADPH

P i

6 P1,3-Bisphosphoglycerate

P

6 PGlyceraldehyde-3-phosphate

(G3P)

P1G3P

(a sugar)Output

Phase 2:Reduction

Glucose andother organiccompounds

3

3 ADP

ATP

Phase 3:Regeneration ofthe CO2 acceptor(RuBP) P5

G3P

9 ATP & 6 NADPH per G3P


5. One good reason for carrying out the production of oxygen gas (O2) in the space surrounded by the thylakoid membranes, and not in the stroma of the chloroplasts, isa. that this makes it easier for O2 to exit the chloroplast.

b. that the hydrogen ions released can contribute to the H electrochemical gradient being generated.

c. to reduce the concentration of O2 in the stroma so that organic matter located there is not oxidized by it.

d. that the concentration of water in this space is high, making it easier to form O2 from the water.

e. that carrying out this process in the stroma would tend to dry out this compartment and denature the enzymes of the Calvin cycle located there.


5. One good reason for carrying out the production of oxygen gas (O2) in the space surrounded by the thylakoid membranes, and not in the stroma of the chloroplasts, isa. that this makes it easier for O2 to exit the chloroplast.

b. that the hydrogen ions released can contribute to the H electrochemical gradient being generated.

c. to reduce the concentration of O2 in the stroma so that organic matter located there is not oxidized by it.

d. that the concentration of water in this space is high, making it easier to form O2 from the water.

e. that carrying out this process in the stroma would tend to dry out this compartment and denature the enzymes of the Calvin cycle located there.


Alternative mechanisms of carbon fixation have evolved in hot, arid climates

• Dehydration is a problem for plants, sometimes requiring tradeoffs with other metabolic processes, especially photosynthesis

• On hot, dry days, plants close stomata, which conserves water but also limits photosynthesis

• The closing of stomata reduces access to CO2

and causes O2 to build up

• These conditions favor a seemingly wasteful process called photorespiration


Photorespiration: An Evolutionary Relic?

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

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

• Photorespiration consumes O2 and organic fuel and releases CO2 without producing ATP or sugar


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

• In many plants, 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


C4 Plants

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

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


Figure 10.20

Mesophyllcell

Bundle-sheath

cell

Photo-synthetic

cells ofC4 plant

leaf

Vein(vascular

tissue)

C4 leaf anatomy

Stoma

The C4 pathway

MesophyllcellPEP carboxylase

Oxaloacetate (4C)

Malate (4C)

Pyruvate(3C)

CO2

ADPPEP (3C)

ATP

CO2

CalvinCycle

Bundle-sheath

cell

Sugar

Vasculartissue


CAM Plants

• 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


Figure 10.21

Sugarcane Pineapple

C4 CO2 CO2 CAM

Organicacid

Organicacid

Night

Day

CO2CO2

CalvinCycle

CalvinCycle

SugarSugar

Bundle-sheath

cell

(a) Spatial separationof steps

(b) Temporal separationof steps

Mesophyllcell

2

1 1

2





LE 10-21

Light

CO2H2O

Light reactions Calvin cycle

NADP+

RuBP

G3PATP

Photosystem IIElectron transport

chainPhotosystem I

O2

Chloroplast

NADPH

ADP+ P i

3-Phosphoglycerate

Starch(storage)

Amino acidsFatty acids

Sucrose (export)


Figure 10.22b

LIGHT REACTIONS CALVIN CYCLE REACTIONS

• Are carried out by moleculesin the thylakoid membranes

• Convert light energy to thechemical energy of ATP

and NADPH• Split H2O and release O2

to the atmosphere

• Take place in the stroma• Use ATP and NADPH to convert

CO2 to the sugar G3P• Return ADP, inorganic phosphate,

and NADP to the light reactions

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Chapter 10: Photosynthesis

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