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Biological Sciences
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Tutorial Photorespiration and CO2 concentrating mechanisms y Found that photosynthetic CO2 uptake in most plants C3 plants was inhibited by O2 even normal atm level of O2 21 was inhibitory The inhibitory effect is greater at lower CO2 y Photosynthesis under these inhibitory conditions correlated by extensive glycolate synthesis in leaf y Explanation CO2 fixing enzyme of photosynthetic carbon reduction cycle Calvin cycle which is Rubisco ribulose 15biphosphate carboxylase also displayed oxygen activityy O2 and CO2 both compete for its active site y The addition of CO2 to RUBP via rubisco yields 2X 3PGA The addition of O2 to RUBP via rubisco yields 1X 3PGA and 1X phosphoglycolate Subsequent removal of Pi produces glycolatey Under present atmospheric conditions 21 O2 0035 CO2 rubisco is fixing 1 O2 for every 3 CO2 fixed y Temperature has an effect on this ratio as temp increases CO2 in solution decreases relatively more than O2 Therefore at higher temps ratio of CO2O2 at Rubiscos active site decreasesmore oxygenase activity y Presence of rubisco oxygenase activitylarge glycolate which must be metabolized occurring in pathway known as Photorespiration y Photorespiration photosynthetic carbon oxidation cycle light dependant process since Rubisco only operates in light which has same gas exchange characteristics as respiration O2 consumption CO2 release y Photorespiration involves 3 separate organelles 1 chloroplast 2 peroxisome 3 mitochondriaMetabolite transfer between these 3 organelles y In photorespiration some of the carbon in glycolate is released as CO2 by mitochondria some is returned to chloroplast as 3PGAy Rubsicos carboxylase activity results in net gain of carbon by plant its oxygenase activity results in net loss of carbon by plant y The photorespiratory pathway is also costly energetic needs ATP and reduced ferredoxin generated by e transport chain are consumed y The photorespiratory pathway involves nitrogen metabolism2 transamination rxns in peroxisome are the first point where nitrogen metabolism enters the cycle transamination rxn amino gr
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