LIGHT-DRIVEN SYNTHESIS OF ATP
ATP is synthesized in PSII
Cytochrome bf pumps protons
Quinones and Plastocyanin are mobile transport agents

-1.3
Pheophytin (ph)
-0.8
Plastoquinones
A o

F
D

A
1

F
DR
F
X

F
A/B

NADP +
Cyto bf complex
Plastocyanin
(Cu 2+ )
NADPH
H +
PS-I
(P700)
PS-II
(P680)

PHOTOSYSTEMS
1. Each photosystem is an electron transport chain
2. Initial acceptor of excited electron has highest reducing potential of the system
3. As electron falls, free energy is made available to pump protons (PSII) or reduce NADP+ (PSI)
4. A water molecule provides the electron (PSII)
5. O
2
(oxidized H
2
O) is the product

1. Absorption maximum is at 700 nm
2. Ferredoxin is recipient NADP +
3. Operates between +0.4 and -1.3 volts
4. Sequence is:
Soluble
A o
A
1
F
X
F
A
/F
B
F
D
NADP +
F
DR Chlorophyll acceptor
Phylloquinone
(Vitamin K
1
)
Iron-Sulfur
Proteins
NADPH
Ferredoxin Reductase

WATER-SPLITTING COMPLEX
E
O
2
2H +
E E
H
2
O
E E
H
2
O
2H +
Mn
O
O
Mn
E
E
O
Mn
Mn
O
E
E
Tyrosine
Z
E
One electron at a time

Light Reaction of PSII
4 P680 + 4H + + 2Q
B
+ 4 Photons
4 P680 + + 4Z
4Z + + [Mn complex]
[Mn complex] 4+ + 2H
2
0
O
4 P680 + + 2Q
B
H
4 P680 + 4Z +
4Z + [Mn complex] 4+
[Mn complex] 0 + 4H + + O
2
2
2H
2
O + 2Q
B
+ 4 Photons O
2
+ 2Q
B
H
2

How many “flashes” (photons) are required to evolve one oxygen molecule
O
2 per flash
0 4 8 12 16 20
Answer: 4

1. PSI is the more primitive system
2. PSI cannot make ATP
3. PSII replaces the electron displaced by PSI
4. PSII gets its electron from H
2
O
5. Z scheme is non-cyclic photophosphorylation

1. Electrons do not go to NADP +
2. Electrons go from F
D

Cyt bf

PC
3. Cyt bf and NADP + compete for electrons
4. NADP + concentration controls the shunt
5. High NADPH/ NADP + ratio favors Cyt bf
6. One ATP for 2 electrons shunted
The purpose of cyclic photophosphorylation is to match ATP levels with NADPH levels to optimize the dark reaction processes.

Photosynthetic Electron Transport System in purple photosynthetic bacteria
One electron carrier
Out of visible range
2 photons are required to reduce Q to QH
2

Some of the energy captured by the photosynthetic systems of chloroplasts is transformed into phosphate bond energy of ATP
Daniel Arnon, 1954
A pH gradient across the thylakoid membrane is capable of furnishing the driving force to generate ATP.
Andre Jagendorf, 1966

O
2 light
2H +
PS
II
Q
QH
2
Cyt bf light
NADP +
PSI
Fd
NADPH
PC
2H
2
O
4H + Lumen
Stroma
ADP + P i
H +
CF o
CF
1
ATP
Proton is pumped out of thylakoid lumen into stroma

Jagendorf’s Experiment
ATPase is oriented out
Chloroplasts
H +
H +
H + pH 4 buffer
CF o
-CF
1
ATPase
No light
H +
H +
H +
H +
H +
H +
H +
ADP + 32 P
Quickly
Raise to pH 8
H +
H +
H +
ADP~ 32 P (ATP)
H +
Chloroplasts synthesize ATP with a proton gradient