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PHOTOPHOSPHORYLATIONLIGHT-DRIVEN SYNTHESIS OF ATP
ATP is synthesized in PSII
Cytochrome bf pumps protons
Quinones and Plastocyanin are mobile transport agents
NADPH
PS-II
PS-I
NADP+
(P680)
(P700)
Pheophytin (ph)
Plastoquinones
Cyto bf complex
H+
-0.8
-1.3
Ao A1 FX FA/B FD FDR
Plastocyanin (Cu2+)
PHOTOSYSTEMS
1. Each photosystem is an electron transport chain
2. Initial acceptor of excited electron has highestreducing 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
Photosystem One (PSI)1. Absorption maximum is at 700 nm
2. Ferredoxin is recipient
3. Operates between +0.4 and -1.3 volts
NADP+
4. Sequence is:
Ao A1 FX FA/FB FD
FDR
NADP+
Chlorophyll acceptor
Phylloquinone
(Vitamin K1)
Iron-SulfurProteins
Soluble
Ferredoxin Reductase
NADPH
WATER-SPLITTING COMPLEX
Z
Mn
Mn
OMn
Mn
O
OO
H2O H2O
E
2H+
O2
2H+
E
E
EE
E
E E E E
One electron at a time
Tyrosine
Equations
4 P680 + 4H++ 2QB + 4 Photons 4 P680+ + 2QBH2
Light Reaction of PSII
4 P680+ + 4Z 4 P680 + 4Z+
4Z+ + [Mn complex]0 4Z + [Mn complex]4+
[Mn complex]4+ + 2H2O [Mn complex]0+ 4H+ + O2
2H2O + 2QB + 4 Photons O2 + 2QBH2
0 4 8 12 16 20
O2
perflash
How many “flashes” (photons) are required to evolve one oxygen molecule
Answer: 4Answer: 4
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
Cyclic Electron Flow1. Electrons do not go to NADP+
2. Electrons go from FD Cyt bf PC
4. NADP+ concentration controls the shunt
3. Cyt bf and NADP+ compete for electrons
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 Systemin purple photosynthetic bacteria
2 photons are required to reduce Q to QH2
Out of visible range
One electron carrier
Arnon’s Observation
Some of the energy captured by the photosyntheticsystems of chloroplasts is transformed intophosphate bond energy of ATP
Daniel Arnon, 1954
Some of the energy captured by the photosyntheticsystems of chloroplasts is transformed intophosphate bond energy of ATP
Daniel Arnon, 1954
Jagendorf’s Observation
A pH gradient across the thylakoid membrane is capable of furnishing the driving force to generate ATP.
Andre Jagendorf, 1966
A pH gradient across the thylakoid membrane is capable of furnishing the driving force to generate ATP.
Andre Jagendorf, 1966
PSII Cyt bf
PCPSI
2H+
2H2O
QQH2
4H+
ADP + PiATP
light
Fd
NADP+
NADPH
light
O2
H+
Proton is pumpedout of thylakoid lumen into stroma
CFo
CF1Stroma
Lumen
Jagendorf’s Experiment
H+
H+
H+
H+ H+
H+H+
pH 4 buffer
H+H+
H+
H+
H+
H+
Chloroplasts
CFo-CF1 ATPase
ADP + 32P
ADP~32P (ATP)
Chloroplasts synthesize ATP with a proton gradient
No light
H+
ATPase isoriented out
QuicklyRaise to pH 8
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