Photosynthesis

Page
7

Light

P680

e–

Primary

acceptor

2

1

e–

e–

2 H+

O2

+

3

H2O

1/2

4

Pq

Pc

Cytochrome

complex

Electron transport chain

5

ATP

Photosystem I

(PS I)

Light

Primary

acceptor

e–

P700

6

Fig. 10-13-4

Photosystem II

(PS II)

Slide 57

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

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

Slide 58

Pigment

molecules

Light

P680

e–

Primary

acceptor

2

1

e–

e–

2 H+

O2

+

3

H2O

1/2

4

Pq

Pc

Cytochrome

complex

Electron transport chain

5

ATP

Photosystem I

(PS I)

Light

Primary

acceptor

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)

Slide 59

Fig. 10-14

Mill

makes

ATP

e–

NADPH

Photon

e–

e–

e–

e–

e–

Photon

ATP

Photosystem II

Photosystem I

e–

Slide 60

Cyclic Electron Flow

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

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

Slide 61

Fig. 10-15

ATP

Photosystem II

Photosystem I

Primary

acceptor

Pq

Cytochrome

complex

Fd

Pc

Primary

acceptor

Fd

NADP+

reductase

NADPH

NADP+

+ H+

Slide 62

Some organisms such as purple sulfur bacteria have PS I but not PS II

Cyclic electron flow is thought to have evolved before linear electron flow

Cyclic electron flow may protect cells from light-induced damage

Slide 63

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

Spatial organization of chemiosmosis differs between chloroplasts and mitochondria but also shows similarities