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Download Photosynthesis: conversion of light energy to chemical energy
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Photosynthesis: conversion of light energy to chemical energy Focus on the light reactions Electron transport system: 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 (3PGA) 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 res t 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. Photosynthesis and respiration are connected • Not all photosynthesis is used to make sugar • Some is needed to reduce nitrate to ammonium 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 + 2 NADP+ + light --> 2S + 2 NADPH + 2 H+ Dark reactions: CO2 + 2 NADPH + H+ --> H2O + CH2O + 2 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 2 lactate + CO2 + light ---> H2O + CH2O + 2 pyruvate Cyanobacteria (aerobic): electron donor = H2O 2 H2O + CO2 + light ---> H2O + CH2O + O2 Rhodopseudomonas viridis: cyclic photophosphorylation H+ eH+ ATP H+ P870 Was photosynthesis an early source of energy?
