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Download Light capture: conversion of light energy to chemical energy
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Light capture: conversion of light energy to chemical energy Focus on the light reactions Electron transport: formation of ATP H+ transport into the thylakoid lumen H+ return through ATP synthase (Called “non-cyclic photophosphorylation” of ADP) Electron transport: formation of ATP: works even if no O2 or NADPH is produced (Called “cyclic photophosphorylation” of ADP) Reduction of CO2 ! Soluble enzyme in stroma (“Rubisco”) adds CO2 to RuBP (a sugar) ! NADPH adds electrons ! Free energy of NADPH oxidation and ATP hydrolysis push the reaction forward Why is cyclic photophosphorylation needed? • Non-cyclic photophosphorylation makes one ATP per NADPH • Calvin cycle uses 18 ATP per 12 NADPH • C yclic photophosphorylation makes the rest of the ATP Photosynthesis and respiration are connected • Not all photosynthate is used to make sugar (although much is temporarily stored as starch). • Not all the citric acid cycle intermediates are completely oxidized. There are several types of bacterial photosynthesis Green bacteria Purple bacteria Cyanobacteria (cyan = bluegreen) Purple and green sulfur bacteria (anaerobic): electron donor = H2S 2 H2S + CO2 + light ---> H2O + CH2O + 2 S Light reactions: 2 H2S + NADP+ light --> 2S + NADPH + H+ Dark reactions: CO2 + NADPH + H+ --> C[H2O] + NADP+ There are several types of bacterial photosynthesis Green bacteria Purple bacteria Cyanobacteria (cyan = bluegreen) Purple and green sulfur bacteria (anaerobic): electron donor = H2S 2 H2S + CO2 + light ---> H2O + CH2O + 2 S Green sulfur bacteria (anaerobic): electron donor = H2 2 H2 + CO2 + light ---> H2O + CH2O Purple nonsulfur bacteria: electron donor = organic lactate + CO2 + light ---> H2O + CH2O + pyruvate Cyanobacteria (aerobic): electron donor = H2O 2 H2O + CO2 + light ---> H2O + CH2O + O2 Rhodopseudomonas viridis: cyclic photophosphorylation H+ e- P870 H+ ATP H+ Will the R. viridis photoreaction center, P870, collect more or less energy per photon than the PSII reaction center of plants (P680)? (a) More (b) Less (c) The same Was photosynthesis an early source of energy?
