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Biology (Sci)
BIOL 201
Tamara Western

L5. Photosynthesis II • The two cooperating photosystems, photosynthetic electron transport, photophosphorylation • Regulation of the light reactions • Two photosystems: cooperate to generate NADPH and proton gradient • PSII • Charge separation: H O t2 O ; Q2-> QH 2 • Transfer of electrons from QH to 2 plastocyanin (Pc) catalyzed by cytochrome b/f complex • Accompanied by movement of H into + lumen • Plastocynanin brings electrons to PSI • PSI • Charge separation: PC redPC oxNADP + NADPH • Linear electron flow acidification of lumen • PSII • Creation of charge separation • Reaction centre: P 680 (max light absorption at 680 nm) • Two specially positioned chlorophyll a molecules • Q (quinone) reduced twice to QH (quin2l) • Oxygen evolution takes four protons • So where in the purple sulphur bacteria, electrons were replenished from cytochrome c, here they come from H O 2 • P + is a powerful oxidant, extracts electrons 680 from H O2bound to a 4-Mn centre • Mn can exist in multiple oxidation states • 2 H O are stripped of 4 electrons by Mn 2 centre of the O -2volving complex to create stable O 2 • Shown experimentally by need for multiples of four short flashes of light for O re2ease • Electron transfer to cytochrome bf complex • Transfer of electrons from QH to 2 plastocyanin (Pc) • Two step process • QH b2ck to Q • PC to PC ox red • Leads to additional proton pumping into thylakoid lumen • Both QH and Pc are mobile electron carriers 2 • Creation of proton gradient (proton motive force) by photosystem II and cytochrome bf complex (slide 13) • Extraction of electrons from H O by PSII leads 2 to evolution of O 2nd production of 2 H + + • QH c2rries another 2 H when it moves to cytochrome bf that are transferred to the lumen when electrons are passed along • Q cycle moves another 2 electrons • PSI • Transfer of electrons to final electron acceptor NADPH • Ph
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