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Photosynthesis Chapter 10

You should be able to:

1. Describe the structure of a chloroplast

2. Describe the relationship between an

action spectrum and an absorption

spectrum

3. Trace the movement of electrons in linear

electron flow

4. Trace the movement of electrons in cyclic

electron flow

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

Cummings

5. Describe the similarities and differences

between oxidative phosphorylation in

mitochondria and photophosphorylation in

chloroplasts

6. Describe the role of ATP and NADPH in

the Calvin cycle

7. Describe the major consequences of

photorespiration

8. Describe two important photosynthetic

adaptations that minimize photorespiration

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Overview: The Process That Feeds

the Biosphere

o Photosynthesis is the process that converts

solar energy into chemical energy (in complex

organic molecules)

o Directly or indirectly, photosynthesis nourishes

almost the entire living world

o Photosynthesis is NOT the direct reverse of

respiration (different pathways), but does store

energy that respiration releases

BioFlix: Photosynthesis

(a) Plants

(b) Multicellular

alga

(c) Unicellular

protists

(d) Cyanobacteria

(e) Purple sulfur

bacteria

10 m

1 m

40 m

Figure 10.2

Photosynthesis occurs in plants,

algae, & some prokaryotes and

protists

o An organism acquires the organic compounds

it uses for energy and carbon skeletons by one

of 2 major modes: autotropic nutrition or

heterotropic nutrition

o Autotrophs sustain themselves -- Without

eating anything derived from other organisms

o Autotrophs are the producers of the biosphere,

producing organic molecules from CO2 and

other inorganic molecules (esp., H2O)

o Almost all plants are photoautotrophs, using

the energy of sunlight to make organic

molecules from H2O and CO2

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o Heterotrophs obtain their organic material

from other organisms

o Animals

o Fungi

o Some Protists

o Some Bacteria

o Heterotrophs are the consumers of the

biosphere

o Almost all heterotrophs, including humans,

depend on photoautotrophs for food and O2

Photosynthesis converts Light energy

to the Chemical energy of food

o Chloroplasts are structurally similar to &

likely evolved from photosynthetic bacteria

o like mitochondria, contain a bit of DNA

o The structural organization of these cells

allows for the chemical reactions of

photosynthesis

Chloroplasts: The Sites of Photosynthesis in Plants

o Leaves are the major locations of

photosynthesis

o Their green color is from chlorophyll, the green

pigment within chloroplasts

o Light energy absorbed by chlorophyll drives the

synthesis of organic molecules in the chloroplast

o CO2 enters & O2 exits the leaf through

microscopic pores called stomata

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o Chloroplasts are found mainly in cells of the

mesophyll, the interior tissue of the leaf

o A typical mesophyll cell has 30–40

chloroplasts

o Chlorophyll is in the membranes of thylakoids

(connected sacs in the chloroplast)

o Thylakoids may be stacked in columns called

grana

o Chloroplasts also contain stroma, a dense fluid

Fig. 10-3a

5 µm

Mesophyll cell

Stomata CO2 O2

Chloroplast

Mesophyll

Vein

Leaf cross section

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Tracking Atoms Through

Photosynthesis

o 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 H2O

Reactants: 6 CO2

Products:

12 H2O

6 O2 6 H2O C6H12O6

The Splitting of Water o Chloroplasts:

o Split H2O into Hydrogen & Oxygen

o Incorporate the Hydrogen into sugar molecules

oRelease oxygen as a by-product

Note: The oxygen

comes from the

splitting of the

water not from

CO2

Photosynthesis as a Redox

Process

Energy + 6 CO2 + 6 H2O → C6H12O6 + 6 O2

Becomes Reduced

Becomes Oxidized

oPhotosynthesis is a redox process in which H2O is oxidized

& CO2 is reduced

oPhotosynthesis is an endergonic process; the energy boost

is provided by light

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The Two Stages of Photosynthesis: A

Preview

o Photosynthesis consists of the light reactions

(the photo part) & Calvin cycle (the synthesis

part)

1. Light Reactions (in the thylakoids):

– Split H2O

– Release O2

– Reduce NADP+→NADPH

– Generate ATP from ADP

by photophosphorylation

2. Calvin cycle (in the stroma) forms sugar

from CO2

--uses ATP & NADPH (from light reactions)

o The Calvin cycle begins with carbon

fixation, incorporating CO2 into organic

molecules

Light

Light Reactions

Chloroplast

NADP

ADP

+ P i

H2O

Figure 10.6-1

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Light

Light Reactions

Chloroplast

ATP

NADPH

NADP

ADP

+ P i

H2O

O2

Figure 10.6-2

Light

Light Reactions

Calvin Cycle

Chloroplast

ATP

NADPH

NADP

ADP

+ P i

H2O CO2

O2

Figure 10.6-3

Light

Light Reactions

Calvin Cycle

Chloroplast

[CH2O] (sugar)

ATP

NADPH

NADP

ADP

+ P i

H2O CO2

O2

Figure 10.6-4

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The Nature of Sunlight

o Light is a form of electromagnetic energy, also

called electromagnetic radiation

o Light also behaves as though it consists of

discrete particles, called photons

o Light travels in rhythmic waves

o Wavelength is the distance

between crests of waves

o Wavelength determines the

type of electromagnetic energy

o The electromagnetic spectrum is the entire range of electro-magnetic energy

o Visible light consists of wavelengths (including those that drive photosynthesis) that produce colors we can see

UV

Fig. 10-6

Visible light

Infrared Micro- waves

Radio waves X-rays

Gamma

rays

103 m 1 m

(109 nm) 106 nm 103 nm 1 nm 10–3 nm 10–5 nm

380 450 500 550 600 650 700 750 nm

Longer wavelength

Lower energy Higher energy

Shorter wavelength

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Reflected light

Absorbed light

Light

Chloroplast

Transmitted light

Granum

Photosynthetic Pigments:

The Light Receptors

o Pigments are substances that absorb visible light

o Different pigments absorb different wavelengths

o Chlorophyll is a pigment

o Wavelengths that are not absorbed are reflected or

transmitted (their color)

o Leaves appear green

because chlorophyll

reflects & transmits

green light (~510-530 nm)

Animation: Light and Pigments

Spectrophotometry (LAB)

o A spectrophotometer measures a

pigment’s ability to absorb/transmit various

wavelengths

o This machine sends light through

pigments & measures the fraction of light

transmitted at each wavelength

Figure 10.9

White light

Refracting prism

Chlorophyll solution

Photoelectric tube

Galvanometer

Slit moves to pass light of selected wavelength.

Green light

High transmittance (low absorption): Chlorophyll absorbs very little green light.

Blue light

Low transmittance (high absorption): Chlorophyll absorbs most blue light.

TECHNIQUE

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o An absorption spectrum is a graph plotting

a pigment’s light absorption vs. wavelength

o The absorption spectrum of chlorophyll a

shows that violet-blue and red light work best

for photosynthesis

o An action spectrum profiles the relative

effectiveness of different wavelengths of

radiation in driving a process

Note: the color at which pigments do NOT work are

the pigment’s color

(b) Action spectrum

(a) Absorption spectra

Chloro- phyll a Chlorophyll b

Carotenoids

Wavelength of light (nm)

Ab

sorp

tion

of

lig

ht

by

ch

loro

pla

st p

igm

ents

R

ate

of

ph

oto

syn

thes

is

(mea

sure

d b

y O

2 r

elea

se)

400 500 600 700

400 500 600 700

RESULTS

o Chlorophyll a -- Main photosynthetic

pigment

o Accessory pigments, such as chlorophyll

b, broaden the spectrum used for

photosynthesis

o Accessory pigments called carotenoids

absorb excessive light that would damage

chlorophyll

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Excitation of Chlorophyll by Light

When a pigment absorbs light:

1. It goes from a ground state to an excited state, which is unstable (when a molecule absorbs a photon of light, 1 of the molecule’s e- is elevated to a orbital with more potential energy, ie farther away from the nucleus)

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

(a) Excitation of isolated chlorophyll molecule

Heat

Excited state

(b) Fluorescence

Photon Ground state

Photon (fluorescence)

En

erg

y o

f ele

ctr

on

e–

Chlorophyll molecule

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

A Photosystem: A Reaction-Center

Complex Associated with Light-

Harvesting Complexes

o A photosystem consists of a reaction-

center complex (a type of protein

complex) surrounded by light-harvesting

complexes

o The light-harvesting complexes

(pigment molecules bound to proteins)

transfer the energy of photons to the

reaction center

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o A primary electron acceptor in the

reaction center accepts excited electrons

and is reduced as a result

o Solar-powered transfer of an electron from

a chlorophyll a molecule to the primary

electron acceptor is the first step of the

light reactions

THYLAKOID SPACE (INTERIOR OF THYLAKOID)

STROMA

e–

Pigment molecules

Photon

Transfer of energy

Special pair of chlorophyll a molecules

Th

yla

ko

id m

em

bra

ne

Photosystem

Primary electron acceptor

Reaction-center complex

Light-harvesting

complexes

1. Energy from

photon

transferred

from pigment

molecule to

pigment

molecule

2. Once the

chlorophyll a

molecules are

excited, an

electron is

transferred to

the primary

electron

acceptor (the

1st step in the

light reaction)

o 2 types of Photosystems in the thylakoid

membrane

o The numbers reflect order of discovery,

opposite their real sequence

o 1st. Photosystem II (PS II) is best at

absorbing a wavelength of 680 nm

o The reaction-center chlorophyll a of PS II is

called P680

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o 2nd. Photosystem I (PS I) is best at

absorbing a wavelength of 700 nm

o The reaction-center chlorophyll a of PS I is

called P700

Linear Electron Flow

o During the light reactions, there are two possible routes for electron flow: Cyclic & Linear

o 1. Linear electron flow, the primary

pathway, involves both photosystems

o Produces ATP & NADPH using light energy

1. A photon hits a pigment

2. Its energy is passed among

pigment molecules until it

excites P680

3. An excited electron from

P680 is transferred to the

primary electron acceptor

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4. P680+ (P680 that is missing an electron) is a very strong oxidizing (very electronegative) agent

5. a. H2O is split by enzymes

b. The electrons are transferred from the hydrogen atoms to P680+, thus reducing it to P680

6. O2 is released as a by-product of this reaction

7. Each electron “falls” down

an electron transport chain

from the primary electron

acceptor of PS II to PS I

o Energy released by the fall

drives the creation of a

proton gradient across the

thylakoid membrane

o Diffusion of H+ (protons)

across the membrane

drives ATP synthesis

o In PS I (like PS II), transferred light energy excites

P700, which loses an electron to an electron

acceptor

o P700+ (P700 that is missing an electron) accepts

an electron passed down from PS II via the

electron transport chain

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o Each electron “falls” down another electron

transport chain from the primary electron acceptor

of PS I to the protein ferredoxin (Fd)

o The electrons are then

transferred to

NADP+ (→ NADPH)

o The electrons of NADPH

are available for the

reactions of

the Calvin cycle

Pigment molecules

Light

P680

e–

Primary acceptor

2

1

e–

e–

2 H+

O2

+

3

H2O

1/2

4

Pq

Pc

Cytochrome complex

5

ATP

Photosystem I (PS I)

Light

Primary acceptor

e–

P700

6

Fd

NADP+ reductase

NADP+

+ H+

NADPH

8

7

e– e–

6

Fig. 10-13-5

Photosystem II (PS II)

Fig. 10-14

Mill

makes

ATP

e–

NADPH

e– e–

e–

e–

e– ATP

Photosystem II Photosystem I

e–

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Cyclic Electron Flow

2. Cyclic electron flow uses only photosystem I

and produces ATP, but not NADPH

o No oxygen is released

o Cyclic electron flow generates surplus ATP,

satisfying the higher demand in the Calvin

cycle

o Some organisms, such as purple sulfur

bacteria, have PS I, but not PS II

o Cyclic electron flow is thought to have

evolved before linear electron flow

o Cyclic electron flow may protect cells from

light-induced damage

Comparing Chemiosmosis

in Chloroplasts & Mitochondria

o Chloroplasts & Mitochondria generate ATP

by chemiosmosis, but use different sources

of energy:

o Mitochondria transfer Chemical energy from

food → ATP

o Chloroplasts transform Light energy into the

chemical energy of ATP

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o Spatial organization of chemiosmosis differs

between chloroplasts & mitochondria, but also

shows similarities:

o Mitochondria--Protons are pumped to the

intermembrane space & drive ATP synthesis

as they diffuse back into the mitochondrial

matrix

o Chloroplasts--Protons are pumped into the

thylakoid space & drive ATP synthesis as they

diffuse back into the stroma

Fig. 10-16

Key

Mitochondrion Chloroplast

CHLOROPLAST

STRUCTURE

MITOCHONDRION

STRUCTURE

Intermembrane

space

Inner

membrane

Electron transport

chain

H+ Diffusion

Matrix

Higher [H+]

Lower [H+]

Stroma

ATP

synthase

ADP + P i

H+ ATP

Thylakoid

space

Thylakoid

membrane

o ATP & NADPH are produced on the side

facing the stroma, where the Calvin cycle

takes place

Summary

Light reactions generate ATP & increase

the potential energy of electrons by

moving them from H2O to NADPH

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Fig. 10-17

Light

Fd

Cytochrome

complex

ADP

+

i H+

ATP P

ATP synthase

To Calvin Cycle

STROMA (low H+ concentration)

Thylakoid membrane

THYLAKOID SPACE (high H+ concentration)

STROMA (low H+ concentration)

Photosystem II Photosystem I

4 H+

4 H+

Pq

Pc

Light NADP+

reductase

NADP+ + H+

NADPH

+2 H+

H2O O2

e– e–

1/2 1

2

3

The Calvin cycle uses ATP and

NADPH to convert CO2 to sugar

o The Calvin cycle (like the citric acid cycle)

regenerates its starting material after

molecules enter & leave the cycle

o The cycle builds sugar from smaller

molecules by using ATP & the reducing

power of electrons carried by NADPH

o Process needs Energy

o Carbon enters the cycle as CO2 and leaves as

a 3-C sugar named glyceraldehyde-3-

phosphate (G3P)

o Can be used to make glucose, or other things, or

stored

o For net synthesis of 1 G3P,the cycle must

take place 3 times, fixing 3 molecules

of CO2

o The Calvin cycle has three phases:

1. Carbon fixation (catalyzed by rubisco)

2. Reduction

3. Regeneration of the CO2 acceptor

(RuBP)

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Input

3 (Entering one at a time)

CO2

Phase 1: Carbon fixation

Rubisco

3 P P

P 6

Short-lived intermediate

3-Phosphoglycerate

3 P P

Ribulose bisphosphate (RuBP)

Figure 10.19-1

Input

3 (Entering one at a time)

CO2

Phase 1: Carbon fixation

Rubisco

3 P P

P 6

Short-lived intermediate

3-Phosphoglycerate 6

6 ADP

ATP

6 P P

1,3-Bisphosphoglycerate

Calvin Cycle

6 NADPH

6 NADP

6 P i

6 P

Phase 2: Reduction

Glyceraldehyde 3-phosphate (G3P)

3 P P

Ribulose bisphosphate (RuBP)

1 P

G3P (a sugar)

Output

Glucose and other organic compounds

Figure 10.19-2

Input

3 (Entering one at a time)

CO2

Phase 1: Carbon fixation

Rubisco

3 P P

P 6

Short-lived intermediate

3-Phosphoglycerate 6

6 ADP

ATP

6 P P

1,3-Bisphosphoglycerate

Calvin Cycle

6 NADPH

6 NADP

6 P i

6 P

Phase 2: Reduction

Glyceraldehyde 3-phosphate (G3P)

P 5

G3P

ATP

3 ADP

Phase 3: Regeneration of the CO2 acceptor

(RuBP)

3 P P

Ribulose bisphosphate (RuBP)

1 P

G3P (a sugar)

Output

Glucose and other organic compounds

3

Figure 10.19-3

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Alternative mechanisms of carbon

fixation have evolved in hot, arid

climates

o Dehydration is a problem for plants, sometimes requiring trade-offs with other metabolic processes, especially photosynthesis

o Hot, dry days: Plants close stomata, which

conserves H2O, but limits photosynthesis

oThe closing of stomata reduces access to

CO2 & causes O2 to build up

o These conditions favor a seemingly wasteful

process called photorespiration

Photorespiration: An Evolutionary

Relic?

o In most plants (C3 plants), initial fixation of

CO2, via rubisco, forms a 3-carbon

compound

o In photorespiration, rubisco adds O2

instead of CO2 in the Calvin cycle

o Photorespiration consumes O2 and organic

fuel & releases CO2 without producing ATP

or sugar

o Photorespiration may be an evolutionary relic

because rubisco first evolved at a time when

the atmosphere had far less O2 and more CO2

o Photorespiration limits damaging products of

light reactions that build up in the absence

of the Calvin cycle

o 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

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C4 Plants

o C4 plants minimize the cost of photorespiration

by incorporating CO2 into 4-carbon compounds

in mesophyll cells

o This step requires the enzyme PEP carboxylase

o PEP carboxylase has a higher affinity for CO2

than rubisco does

o It can fix CO2 even when CO2 concentrations are

low

o These 4-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 cell Photosynthetic cells of C4 plant leaf

Bundle- sheath cell

Vein (vascular tissue)

Stoma

The C4 pathway

Mesophyll cell CO2

PEP carboxylase

Oxaloacetate (4C)

Malate (4C)

PEP (3C)

ADP

ATP

Pyruvate (3C)

CO2 Bundle- sheath cell

Calvin Cycle

Sugar

Vascular tissue

CAM Plants

Some plants use crassulacean acid

metabolism (CAM) to fix carbon

1. CAM plants open their stomata at night,

incorporating CO2 into organic acids

2. Stomata close during the day, & CO2 is

released from organic acids and used in the

Calvin cycle

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Fig. 10-20

CO2

Sugarcane

Mesophyll cell

CO2

C4

Bundle- sheath cell

Organic acids release CO2 to Calvin cycle

CO2 incorporated into four-carbon organic acids (carbon fixation)

Pineapple

Night

Day

CAM

Sugar Sugar

Calvin

Cycle

Calvin Cycle

Organic acid Organic acid

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

CO2 CO2

1

2

In C4 plants,

carbon fixation

and the Calvin

cycle occur in

different types

of cells

In CAM

plants,

carbon

fixation and

the Calvin

Cycle occur

in the same

cells but at

different

times

Importance of Photosynthesis: A Review

o The energy entering chloroplasts as sunlight

gets stored as chemical energy in organic

compounds

o Sugar made in the chloroplasts supplies

chemical energy & carbon skeletons to

synthesize the other organic molecules of

cells

o Plants store excess sugar as starch in

structures such as roots, tubers, seeds, and

fruits

o Photosynthesis also produces the O2 in our

atmosphere