H2o is split by enzymes and the electrons are

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H2O is split by enzymes, and the electrons are transferred from thehydrogen atoms to P680+, thus reducing it to P680vi.O2 is released as a by-product of this reactionvii.Each electron “falls” down an electron transport chain from the primaryelectron acceptor of PS II to PS Iviii.Energy released by the fall drives the creation of a proton gradientacross the thylakoid membraneix.Diffusion of H+ (protons) across the membrane drives ATP synthesisx.In PS I (like PS II), transferred light energy excites P700, which loses anelectron to an electron acceptorxi.P700+ (P700 that is missing an electron) accepts an electron passeddown from PS II via the electron transport chainxii.Each electron “falls” down an electron transport chain from the primaryelectron acceptor of PS I to the protein ferredoxin (Fd)b.Cyclic Electron Flowi.Cyclic electron flow uses only photosystem Iand produces ATP, butnot NADPH55
Biological Science ITuesdays and Thursdays 9:30-10:45, WJB 2004Wednesdays 5:15-6:15, KIN 1024BSC 2010Dr. Dennisii.Cyclic electron flow generates surplus ATP, satisfying the higherdemand in the Calvin cycleiii.Some organisms such as purple sulfur bacteria have PS I but not PS IIiv.Cyclic electron flow is thought to have evolved before linear electronflowv.Cyclic electron flow may protect cells from light-induced damage56
Biological Science ITuesdays and Thursdays 9:30-10:45, WJB 2004Wednesdays 5:15-6:15, KIN 1024BSC 2010Dr. Dennis12.The Calvin cycle uses ATP and NADPH to convert CO2to sugara.The Calvin cycle, like the citric acid cycle, regenerates its starting material aftermolecules enter and leave the cycleb.The cycle builds sugar from smaller molecules by using ATP and the reducingpower of electrons carried by NADPHc.Carbon enters the cycle as CO2and leaves as a sugar named glyceraldehyde-3-phospate (G3P)d.For net synthesis of 1 G3P, the cycle must take place three times, fixing 3molecules of CO2e.The Calvin cycle has three phases:i.Carbon fixation (catalyzed by rubisco)ii.Reductioniii.Regeneration of the CO2acceptor (RuBP)RUBISCO CAN BIND O2 WITH CATLYTIC EFFICIENCY13.Alternative mechanisms of carbon fixation have evolved in hot, arid climatesa.Dehydration is a problem for plants, sometimes requiring trade-offs with othermetabolic processes, especially photosynthesisb.On hot, dry days, plants close stomata, which conserves H2O but also limitsphotosynthesisc.The closing of stomata reduces access to CO2and causes O2to build upd.These conditions favor a seemingly wasteful process called photorespirationi.In most plants (C3 plants), initial fixation of CO2, via rubisco, forms athree-carbon compoundii.In photorespiration, rubisco adds O2instead of CO2in the Calvin cycleiii.Photorespiration consumes O2and organic fuel and releases CO2without producing ATP or sugariv.Photorespiration may be an evolutionary relic because rubisco firstevolved at a time when the atmosphere had far less O2and more CO2v.Photorespiration limits damaging products of light reactions that build

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