Chapter ThreePhotosynthesisIII MechanismsofphotosynthesisPhotosynthesis include three steps:(1) Primary reactionPigment molecules (All Chlb, 98-99% Chla, all carotenoid.)absorb Light energy, transfer Light energy and transformation light energy Light reaction.·Location:thylakoid membrane(2)Electron transport, proton transport and photophosphorylation.(formation ATP andNADPH.)?Light reaction·Thylakoidmembrane(3）Assimilation ofcarbondioxideC3——pathway or called Calvin Cycle.C——pathway or called C,——dicarboxylic acid pathwayCAM pathway? Dark reaction:Chloroplast stroma.1、Primaryreaction(1) Light-Harvesting Pigment System (or called antenna pigment system)Includeall Chlb,98-99%Chlaand carotenoids（2）Reactioncentre①Primary electron acceptor A(Pheo,Ao)PrimaryelectrondonorD (Tyr, PC)Reaction center pigment P680(Chla dimmer),P700(Chladimmer)h→D.P*.A-(3) D.PA-→D.P+.A- -→ D.P.A1
1 Chapter Three Photosynthesis Ⅲ Mechanisms of photosynthesis Photosynthesis include three steps: （3） Assimilation of carbon dioxide C3——pathway or called Calvin Cycle. C4——pathway or called C4——dicarboxylic acid pathway. CAM pathway ·Dark reaction: Chloroplast stroma. 1、Primary reaction （1）Light-Harvesting Pigment System (or called antenna pigment system ) Include all Chlb, 98-99%Chla and carotenoids. （2）Reaction centre ①Primary electron acceptor A(Pheo ,Ao) ②Primary electron donor D (Tyr , PC ) ③Reaction center pigment P680(Chla dimmer), P700(Chla dimmer) （3）D.P.A ⎯⎯→hr D.P*.A ⎯⎯→ D.P+ .A- ⎯⎯→ D.P.A- (1) Primary reaction Pigment molecules (All Chlb, 98-99% Chla, all carotenoid.) absorb Light energy, transfer Light energy and transformation light energy. ·Light reaction. ·Location: thylakoid membrane (2)Electron transport, proton transport and photophosphorylation. (formation ATP and NADPH.) ·Light reaction ·Thylakoid membrane
photosyntheticunitDReaction CentreNADP+Figure 3.1Primaryreaction2.Electrontransport,protontransportandphotophosphorylationAFes,FeS+Fes6Fd.ANADP0.5NADPHavtochretleCytbCytb0FeS:CytLighs1.P7004iqhtDRRO,+H*Photosystem!Photosystem.IFigure 3.2Electron transportchainof photosynthesis("z scheme")2
2 Figure 3.1 Primary reaction 2. Electron transport, proton transport and photophosphorylation. Figure 3.2 Electron transport chain of photosynthesis(“z scheme”) Reaction Centre photosynthetic unit
①Noncyclicelectron transportH2OOEC-TyrP680-Phe0QA-QB→PQ-Cytbof-PCP700-Ao-A1Fe-Sx-Fe-SA·B-Fd-FPNADP+Cyclicelectron transportPC-P700-Ao-A,Fe-Sx-Fe-SA-B-Fd-Cytbaf-PQ-PC· Photosystem IIoxidizes water to O2 in the thylakoid lumen and in the process release protoninto the lumen OEC(oxygen-evolving complex have 5 different forms, So、St、S2、S: and S4.Theirroles are oxidation water.Water is a very stable molecule.Oxidation of water to form molecularoxygen is very difficult.)·Cytochrome b6f receiveselectronsfromPsII and deliversthem toPSI.It alsotransportsadditional proton into the lumen from the stroma.·Photosystem I reduces NADP to NADPH in the stroma by the action of ferredoxin(Fd) and theflavin protein ferredoxin-NADPreductase (FNR or called Fp)。③Electronflowthroughthecytochromebofcomplexalsotransportprotons.3
3 ①Noncyclic electron transport H2O→OEC→Tyr→P680→Pheo→QA→QB→PQ→Cytb6f→PC→P700→A0→A1→Fe-Sx→ Fe-SA·B→Fd ⎯⎯→FP NADP+ ②Cyclic electron transport PC→P700→A0→A1→Fe-Sx →Fe-SA·B→Fd→Cytb6f→PQ→PC ·Photosystem Ⅱoxidizes water to O2 in the thylakoid lumen and in the process release proton into the lumen. OEC (oxygen-evolving complex have 5 different forms, S0、S1、S2、S3 and S4 .Their roles are oxidation water. Water is a very stable molecule.Oxidation of water to form molecular oxygen is very difficult.) ·Cytochrome b6 f receives electrons from PSⅡ and delivers them to PSI. It also transports additional proton into the lumen from the stroma. ·Photosystem I reduces NADP to NADPH in the stroma by the action of ferredoxin(Fd) and the flavin protein ferredoxin-NADP reductase (FNR or called Fp)。 ③ Electron flow through the cytochrome b6f complex also transport protons
(A)First QH2oxidizedCytochromebf complexSTROMAQeQ:ThylakoidmembraneCytbQH2PSICytb-P700PSIIeQ4FeSReCytfePlastocyanin2H+LUMEN(B)SecondQH2oxidizedSTROMACytochromebgf complexQ:2 H+?ThylakoidmembraneCyt bAeQH2+PSICytb.-P700+QH2PSIIOAQ4FeSReCytfePlastocyanin2H+LUMENFigure 3.33Mechanismofelectronandprotontransferinthecytochromebofcomplex④ProtontransportandATP synthesis inthechloroplastPhotophosphorylation concept:The formation of ATP from ADP and inorganic phosphate (p:)usinglight energystored intheprotongradientacross thethylakoidmembrane>
4 Figure 3.3 Mechanism of electron and proton transfer in the cytochrome b6f complex. ④ Proton transport and ATP synthesis in the chloroplast Photophosphorylation concept: The formation of ATP from ADP and inorganic phosphate (pi) using light energy stored in the proton gradient across the thylakoid membrane
HADP+NADPNADPHLightFNRATPsynthasbgPlastoquinoneHighElectrochemicalPlastocyanirH,OHpotentialO,+HgradientOxidationofwaterLUMEN (highH*)Fignre3.4Mechanismsofphotophosphorylation·Chemiosmotic mechanism, First proposed in the 1960s by Peter Mitchell. Mitchell proposed thatthe total energy availablefor ATP synthesis, which he called the proton motiveforce (P), is thesumofaprotonchemical potential andatransmembraneelectricpotentialThestoichiometryof protonstranslocatedperATPsynthesizedhasrecentlybeenfoundtobefour H+ ions per ATP.·StructureofATPsynthaseSTROMAHATPEADP+?CF.ThylakoidmembraneHLUMENFigure 3.5StructureofATPsynthase
5 Fignre 3.4 Mechanisms of photophosphorylation. ·Chemiosmotic mechanism, First proposed in the 1960s by Peter Mitchell. Mitchell proposed that the total energy available for ATP synthesis, which he called the proton motive force (△P), is the sum of a proton chemical potential and a transmembrane electric potential. · The stoichiometry of protons translocated per ATP synthesized has recently been found to be four H+ ions per ATP. ·Structure of ATP synthase Figure 3.5 Structure of ATP synthase