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