CIE A Level Biology (9700) 2019-2021

Revision Notes

13.1.7 The Calvin Cycle

Stages of the Calvin Cycle

  • Energy from ATP and hydrogen from reduced NADP are passed from the light-dependent stage to the light-independent stage of photosynthesis
  • The energy and hydrogen are used during the light-independent reactions (known collectively as the Calvin cycle) to produce 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)
  • This stage of photosynthesis does not, in itself, require energy from light (hence light-independent) and can therefore take place in light or darkness. However, as it requires inputs of ATP and reduced NADP from the light-dependent stage, the Calvin cycle 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 convert 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:
    • Triose phosphates can condense to become hexose phosphates (6C), which can be used to produce starch, sucrose or cellulose
    • Triose phosphates can be converted to glycerol and glycerate 3-phosphates to fatty acids, which join to form lipids for cell membranes
    • Triose phosphates can be used in the production of amino acids for protein synthesis

Regeneration of ribulose bisphosphate

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

Calvin Cycle Intermediates

  • Intermediate molecules of the Calvin cycle (such as glycerate 3-phosphate and triose phosphate) are used to produce other molecules
  • Glycerate 3-phosphate (GP) is used to produce some amino acids
  • Triose phosphate (TP) is used to produce:
    • Hexose phosphates (6C), which can be used to produce starch, sucrose or cellulose
    • Lipids for cell membranes
    • Amino acids for protein synthesis

Author:

Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.
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