Photosynthesis: light-independent reactions

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In the light independent reactions, carbon dioxide is first fixed and then reduced to carbohydrate, using the ATP and reduced NADP produced in the light dependent reactions. ATP supplies the necessary energy, and reduced NADP supplies the hydrogen atoms needed for reduction. The reactions involved are cyclic: Calvin Cycle.

  • Carbon dioxide, which has diffused into the stroma of a chloroplast, is combined with an acceptor molecule that is a 5-carbon sugar, ribulose bisphosphate (RuBP)
  • The enzyme needed for this is ribulose bisphosphate carboxylase (rubisco)
  • An unstable 6-carbon intermediate molecule breaks down into two molecules of the 3-carbon compound glycerate 3-phosphate (GP) . Plants in which GP is the first detectable compound to include the newly fixed carbon dioxide are called C3 plants (because GP is a 3-carbon compound)
  • Using ATP from photophosphorylation and hydrogen from reduced NADP, both from the light-dependent reactions, the GP is reduced to triose phosphate , a 3-carbon sugar. (Some GP is converted to glycerol and amino acids). Nitrogen and sulfur are also needed if amino acids, and hence

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