Chapter 8Light Reactions

Need To Know• How photosystems convert light energy into

chemical energy. (There will be more on this in the next couple of days.)

• How photosystems convert solar energy to chemical energy.

• How linear electron flow in the light reactions results in the formation of ATP, NADPH, and O2

• How the formation of a proton gradient in the light reactions is used to for ATP from ADP and inorganic phosphate by ATP synthase.

Figure 8.11

Photon(fluorescence)

Groundstate

(b) Fluorescence

Excitedstate

Chlorophyllmolecule

Photon

Heat

e−

(a) Excitation of isolated chlorophyll molecule

En

erg

y o

f el

ectr

on

Figure 8.14

Photosystem II Photosystem I

NADPH

Millmakes

ATP

Ph

oto

n

Ph

oto

n

Figure 8.UN02

CalvinCycle

NADPH

NADP

ATP

ADP

Light

CO2

[CH2O] (sugar)

LightReactions

O2

H2O

Figure 8.12a

(a) How a photosystem harvests light

STROMA

THYLAKOID SPACE(INTERIOR OF THYLAKOID)

PhotonPrimaryelectronacceptor

Special pair ofchlorophyll amolecules

Transferof energy

Pigmentmolecules

Th

ylak

oid

mem

bra

ne

e−

Photosystem

Light-harvestingcomplexes

Reaction-centercomplex

There are two reaction centers

P680 P700

Linear electron flow

Figure 8.13-2

Photosystem II(PS II)

P680

Pigmentmolecules

Light

1

Primaryacceptor

22 H

O2

H2O

2

1 3

e−

e−

e−

ATP

Pq

Electrontransportchain

Cytochromecomplex

Pc

4

5Light

6

Pc

Primaryacceptor

Photosystem I(PS I)

P700

Light

6

NADPH

HNADP

reductase

NADP8e−

Fd

Electrontransportchain

7

Figure 8.16

Photosystem II Photosystem I

ToCalvinCycle

H

THYLAKOID SPACE(high H concentration)

Thylakoidmembrane

STROMA(low H concentration)

ATPsynthase

NADPH

e−

LightNADP

ATPADP

NADP

reductase

FdH

Pq

Pc

Cytochromecomplex

4 H

Light

2 HO2

H2O21

4 H

e−

1

2

3

Pi