PHOTOPHOSPHORYLATION LIGHT-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+H A o  A 1  F X  F A/B  F D  F DR Plastocyanin (Cu 2+ )

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

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: AoAo A1A1 FXFX F A /F B FDFD F DR NADP + Chlorophyll acceptor Phylloquinone (Vitamin K 1 ) Iron-Sulfur Proteins Soluble Ferredoxin Reductase NADPH

WATER-SPLITTING COMPLEX Z Mn O O O O H2OH2OH2OH2O E 2H + O2O2 E E E E E EEEE One electron at a time Tyrosine

Equations 4 P H + + 2Q B + 4 Photons4 P Q B H 2 Light Reaction of PSII 4 P Z 4 P Z + 4Z + + [Mn complex] 0 4Z + [Mn complex] 4+ [Mn complex] H 2 O[Mn complex] 0 + 4H + + O 2 2H 2 O + 2Q B + 4 Photons O 2 + 2Q B H 2

O 2 per flash How many “flashes” (photons) are required to evolve one oxygen molecule Answer: 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 H 2 O 5. Z scheme is non-cyclic photophosphorylation

Cyclic Electron Flow 1. Electrons do not go to NADP + 2. Electrons go from F D  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 System in purple photosynthetic bacteria 2 photons are required to reduce Q to QH 2 Out of visible range One electron carrier

Arnon’s Observation Some of the energy captured by the photosynthetic systems of chloroplasts is transformed into phosphate bond energy of ATP Daniel Arnon, 1954 Some of the energy captured by the photosynthetic systems of chloroplasts is transformed into phosphate 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

PS II Cyt bf PC PSI 2H + 2H 2 O Q QH 2 4H + ADP + P i ATP light Fd NADP + NADPH light O2O2 H+H+ Proton is pumped out of thylakoid lumen into stroma CF o CF 1 Stroma Lumen

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