The light-independent reactions are the second stage of photosynthesis. They take place in the stroma and use the ATP and reduced NADP made in the light-dependent reactions to convert carbon dioxide into carbohydrate. They are also known as the Calvin cycle, after Melvin Calvin, who worked out the pathway.They do not use light directly, but they do depend on light, because they need a constant supply of ATP and reduced NADP. In the dark these run out within seconds, and the light-independent reactions soon stop.In outline, three things happen:
• Fixation: carbon dioxide combines with a 5-carbon acceptor, RuBP, catalysed by the enzyme RUBISCO. This forms GP.
• Reduction: GP is reduced to a 3-carbon sugar, GALP, using ATP and reduced NADP.
• Regeneration: most of the GALP is used to remake RuBP, so more CO2 can be fixed. The rest is used to make glucose and other organic molecules.
Every step is catalysed by enzymes in the stroma, so the rate is strongly affected by temperature, as well as by carbon dioxide concentration.Applying it: if the light is switched off, ATP and reduced NADP run out. GP can still be made from RuBP and CO2, but it cannot be reduced, so the concentration of GP rises. RuBP is still used up but cannot be regenerated, so RuBP falls. If instead the CO2 supply is cut, the opposite happens: RuBP rises and GP falls.
• Fixation: carbon dioxide combines with a 5-carbon acceptor, RuBP, catalysed by the enzyme RUBISCO. This forms GP.
• Reduction: GP is reduced to a 3-carbon sugar, GALP, using ATP and reduced NADP.
• Regeneration: most of the GALP is used to remake RuBP, so more CO2 can be fixed. The rest is used to make glucose and other organic molecules.
Every step is catalysed by enzymes in the stroma, so the rate is strongly affected by temperature, as well as by carbon dioxide concentration.Applying it: if the light is switched off, ATP and reduced NADP run out. GP can still be made from RuBP and CO2, but it cannot be reduced, so the concentration of GP rises. RuBP is still used up but cannot be regenerated, so RuBP falls. If instead the CO2 supply is cut, the opposite happens: RuBP rises and GP falls.