PHOTOPHOSPHORYLATION

1
PHOTOPHOSPHORYLATION
LIGHT-DRIVEN SYNTHESIS OF ATP
ATP is synthesized in PSII
Cytochrome bf pumps protons
Quinones and Plastocyanin are mobile transport
agents
2
-1.3
-0.8
NADP
NADPH
(P700)
PS-I
(P680)
PS-II
3
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. O2 (oxidized H2O) is the product
4
Photosystem One (PSI)
1. Absorption maximum is at 700 nm
NADP
2. Ferredoxin is recipient
3. Operates between 0.4 and -1.3 volts
4. Sequence is
Ao
A1
FX
FA/FB
FD
NADP
FDR
Iron-Sulfur Proteins
NADPH
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WATER-SPLITTING COMPLEX
O2
H2O
H2O
2H
2H
Tyrosine
Z
One electron at a time
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Equations
Light Reaction of PSII
2H2O 2QB 4 Photons
O2 2QBH2
7
How many flashes (photons) are required to
evolve one oxygen molecule
O2 per flash
0 4 8 12 16 20
Answer 4
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Non-Cyclic Electron Flow
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 H2O
5. Z scheme is non-cyclic photophosphorylation
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Cyclic Electron Flow
1. Electrons do not go to NADP
2. Electrons go from FD ? 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.
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Photosynthetic Electron Transport System in
purple photosynthetic bacteria
2 photons are required to reduce Q to QH2
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Arnons Observation
Some of the energy captured by the
photosynthetic systems of chloroplasts is
transformed into phosphate bond energy of
ATP Daniel Arnon, 1954
Jagendorfs Observation
A pH gradient across the thylakoid membrane is
capable of furnishing the driving force to
generate ATP. Andre Jagendorf, 1966
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light
light
2H
NADP
Fd
PS II
Q
NADPH
PSI
Cyt bf
QH2
PC
2H2O
4H
Lumen
O2
CFo
Stroma
Proton is pumped out of thylakoid lumen into
stroma
CF1
ATP
ADP Pi
H
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Jagendorfs Experiment
pH 4 buffer
No light
H
H
H
H
H
H
H
ADP 32P
Quickly Raise to pH 8
ADP32P (ATP)
Chloroplasts synthesize ATP with a proton gradient