OCR A Level Biology

Revision Notes

5.6.7 The Light-Independent Stage

The Light-Independent Stage

  • The light-independent stage of photosynthesis is sometimes referred to as the Calvin cycle
  • This stage produces complex organic molecules, including (but not limited to) carbohydrates, such as:
    • Starch (for storage)
    • Sucrose (for translocation around the plant)
    • Cellulose (for making cell walls)
  •  The light-independent stage does not require energy from light and can therefore take place in light or darkness.
  • However, as it requires inputs of ATP and reduced NADP from the light-dependent stage, it cannot continue indefinitely in darkness, as these inputs will run out
  • There are three main steps within the Calvin cycle:
    • Rubisco catalyses the fixation of carbon dioxide by combination with a molecule of ribulose bisphosphate (RuBP), a 5C compound, to yield two molecules of glycerate 3-phosphate (GP), a 3C compound
    • GP is reduced to triose phosphate (TP) in a reaction involving reduced NADP and ATP
    • RuBP is regenerated from TP in reactions that use ATP

Carbon fixation

  • Carbon dioxide combines with a five-carbon (5C) sugar known as ribulose bisphosphate (RuBP)
  • An enzyme called rubisco (ribulose bisphosphate carboxylase) catalyses this reaction
  • The resulting six-carbon (6C) compound is unstable and splits in two
  • This gives two molecules of a three-carbon (3C) compound known as glycerate 3-phosphate (GP)
  • The carbon dioxide has been ‘fixed’ (it has been removed from the external environment and has become part of the plant cell)
  • Glycerate 3-phosphate (GP) is not a carbohydrate but the next step in the Calvin cycle converts it into one

Reduction of glycerate 3-phosphate

  • Energy from ATP and hydrogen from reduced NADP – both produced during the light-dependent stage of photosynthesis – are used to reduce glycerate 3-phosphate (GP) to a phosphorylated three-carbon (3C) sugar known as triose phosphate (TP)
  • One-sixth of the triose phosphate (TP) molecules are used to produce useful organic molecules needed by the plant

Regeneration of ribulose bisphosphate

  • Five-sixths of the triose phosphate (TP) molecules are used to regenerate ribulose bisphosphate (RuBP)
  • This process requires ATP

The Calvin cycle, downloadable AS & A Level Biology revision notes

The Calvin cycle.

Exam Tip

Make sure to learn when in the Calvin cycle ADP and NADP are produced as you will be expected to know it!


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