Photosynthesis

Page
6

Slide 47

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

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

Slide 48

Linear Electron Flow

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

Linear electron flow, the primary pathway, involves both photosystems and produces ATP and NADPH using light energy

Slide 49

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

An excited electron from P680 is transferred to the primary electron acceptor

Slide 50

Pigment

molecules

Light

P680

e–

2

1

Fig. 10-13-1

Photosystem II

(PS II)

Primary

acceptor

Slide 51

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

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

O2 is released as a by-product of this reaction

Slide 52

Pigment

molecules

Light

P680

e–

Primary

acceptor

2

1

e–

e–

2 H+

O2

+

3

H2O

1/2

Fig. 10-13-2

Photosystem II

(PS II)

Slide 53

Each electron “falls” down an electron transport chain from the 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

Slide 54

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

Fig. 10-13-3

Photosystem II

(PS II)

Slide 55

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

Slide 56

Pigment

molecules