Calvin Cycle (Light-Independent Reactions)

• The Calvin cycle


  The second stage of photosynthesis, also called the light-independent reactions, where carbon dioxide is fixed into organic molecules
• Reactions of the Calvin cycle

  1. Carbon fixation

  2. Reduction

  3. Regeneration of the starting molecule
• Carbon fixation

  A CO2 molecule combines with a five-carbon acceptor molecule (RuBP), making a six-carbon compound that splits into two three-carbon 3-PGA molecules. This is catalyzed by the enzyme rubisco.
• Reduction

  ATP and NADPH are used to convert the 3-PGA molecules into the three-carbon sugar G3P
• Regeneration

  Some G3P molecules go to make glucose, while others must be recycled to regenerate the RuBP acceptor. This requires ATP.
• In order for one G3P to exit the cycle and go towards glucose synthesis, three CO2 molecules must enter the cycle.
• Summary of Calvin cycle reactants and products

  • 3 CO2 combine with 3 RuBP, making 6 G3P

  1 G3P exits the cycle, 5 G3P are recycled to regenerate 3 RuBP

  9 ATP are converted to 9 ADP

  6 NADPH are converted to 6 NADP+
• It would take six turns of the Calvin cycle, or 6 CO2, 18 ATP, and 12 NADPH, to produce one molecule of glucose because one cycle takes in one CO2
• You need 2 G3P molecules to make one glucose