Class 11 Biology Photosynthesis In Higher Plants The Calvin cycle

The Calvin cycle

  • The Calvin pathway occurs in all photosynthetic plants.
  • The Calvin cycle can be described under three stages
  • Carboxylation
  • reduction and
  • Carboxylation
  • Carboxylation is the fixation of CO2 into a stable organic intermediate, where CO2 is utilised for the carboxylation of RuBP.
  • Carboxylation is catalysed by the enzyme RuBP carboxylase, which results in the formation of two molecules of 3-PGA.
  • Since this enzyme also has an oxygenation activity it is called as RuBP carboxylase-oxygenase or RuBisCO.
  • Reduction
  • The steps involve utilisation of 2 molecules of ATP for phosphorylation and 2 of NADPH for reduction per CO2 molecule fixed.
  • The fixation of six molecules of CO2 and 6 turns of the cycle are required for the removal of one molecule of glucose from the pathway.
  • Regeneration
  • The regeneration steps require one ATP for phosphorylation to form RuBP.
  • Regeneration of the CO2 acceptor molecule RuBP is crucial if the cycle is to continue uninterrupted.
  • For every CO2 molecule entering the Calvin cycle, 3 molecules of ATP and 2 of NADPH are required.
  • To make one molecule of glucose, 6 turns of the cycle are required.



Fig. Calvin cycle

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