Lecture_No_7-Photosynthesis_II

Lecture_No_7-Photosynthesis_II - The Photosynthetic Steps...

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Unformatted text preview: The Photosynthetic Steps Chloroplast synthesize ATP after generation of pH gradient Proton motive force (p) from pH gradient generates ATP - Peter Mitchell Two components: a charge gradient and a chemical gradient In mitochondrion, membrane potential plays a major role. In chloroplast, nearly all p is from pH gradient. Because, the thylakoid membrane is quite permeable to Cl- and Mg 2+ . ex Mg 2+ (out) and Cl- (in) upon transfer of H + into lumen This is in contrast to mitochondria inner membrane Electrical neutrality is maintained and no membrane potential is generated. Chloroplast ATP synthase: CF1-CF0 complex Closely resembles F1-F0 complex of mitochondria. CF0 conducts protons across the thylakoid membr. CF1 catalyzes ATP formation. CF0: embedded in the thylakoid membrane has four polypeptides: I, II, III and IV with a ratio of 1:2:12:1. I and II similar to subunit b of mito F0. III corresponds to subunit c of mito F0. IV is similar in sequence to subunit a of F0. CF1: the site of ATP sythase has composition 33. contains the catalytic site, 60% identical in amino acid sequence with those of human ATP synthase, despite the passage of 1 billion years since the separation of the plant and animal. CF1-CF0 membrane orientation is reversed. ATP is generated in stroma ATP is generated in matrix Comparison of Chemiosomosis in Chloroplast and Mitochondria FEATURE CHLOROPLAST MITOCHONDRIA membrame involved thylakoid cristae High H + thylakoid Space Intermembrane space Hydrogen/ Electron donor H 2 O NADH and Succinate Final electron/hydrogen acceptor NADP to form NADPH O 2 to form H 2 Nature of reactions Light dependent reaction ETS of aerobic respiration Where ATP is formed stroma matrix Cyclic electron flow through Photosystem I leads to production of ATP instead of NADPH When the ratio of NADPH to NADP + is high, NADP+ is unable to accept electrons from reduced ferredoxin. Instead, the electron in reduced ferredoxin is transferred to the cytochrome bf complex. Then, the e travels to plastocyanin, which can be reoxidized by P700 + to complete a cycle. The net outcome of the cyclic flow of e is pumping protons by cytochrome bf complex e travels from Fd red to cyto. bf to Pc Reduced Pc is reoxidized by P700+ Cyclic photo phosphorylation Cyclic Electron Transport The resulting proton gradient from cyclic electron transport drives ATP synthesis . The process is called Cyclic photophosphorylation, ATP is generated without NADPH formation. H 2 0 is not making Oxygen PS II is not working Leads to the synthesis of a higher amount of ATP Overall stoichiometry of the light reactions-- Photosystem II needs to absorb 4 photons to generate 1 O 2 ....
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Lecture_No_7-Photosynthesis_II - The Photosynthetic Steps...

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