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Light-Independent Reactions

Biology: Cambridge International A Level, topic 13.1.3

The Calvin Cycle

The light-independent reaction (the Calvin cycle) takes place in the stroma of the chloroplast. This reaction uses ATP, reduced NADP, CO2 and ribulose bisphosphate to produce triose phosphates.

1) Carbon fixation

1) Carbon fixation

  • When CO2 diffuses into the leaves through stomata, it combines with ribulose bisposphate (RuBP) to give an unstable 6-carbon compound.
  • This is the first reaction in the Calvin cycle and it is catalysed by an enzyme, RuBisCO.
  • The unstable 6-carbon molecule rapidly splits into two molecules of glycerate 3-phosphate (GP).
  • The combination of CO2 with RuBP is described as carbon fixation.
2) Reduction of glycerate 3-phosphate

2) Reduction of glycerate 3-phosphate

  • The two molecules of GP are then reduced to triose phosphates (TP).
  • This reaction is driven by energy from two molecules of ATP and protons from two molecules of reduced NADP.
  • The ATP and reduced NADP are provided by the light-dependent reaction and are recycled after they have been used.
  • One out of every six TP molecules is converted into hexose sugars but most continue in the Calvin cycle.
3) Regeneration of RuBP

3) Regeneration of RuBP

  • Five out of every six TP molecules are not converted into sugars. These are used to regenerate RuBP.
  • This reaction is driven by one molecule of ATP which is then recycled in the light-dependent reaction.
  • The formation of RuBP from TP allows the Calvin cycle to continue.

Products of the Calvin Cycle

Triose phosphate (TP) and glycerate 3-phosphate (GP) molecules produced in the Calvin cycle are converted into useful organic substances. These are essential for plant survival.

Carbohydrates

Carbohydrates

  • Hexose sugars (monomers) are produced from two molecules of triose phosphate.
  • E.g. Glucose.
  • Hexose sugars can be joined together to form larger carbohydrates (polymers).
  • E.g. Starch, cellulose.
Lipids

Lipids

  • Lipids are made from glycerol and fatty acid chains.
  • Both components of lipids are synthesised from the products of the Calvin cycle:
  • Triose phosphates are used in the synthesis of glycerol.
  • Fatty acids are formed from glycerate 3-phosphate.
Amino acids

Amino acids

  • Glycerate 3-phosphate is used in the synthesis of some amino acids.

Test yourself on Light-Independent Reactions

  • Which of the following statements about the Calvin cycle are true?

  • Which of these molecules are NOT invovled in the Calvin cycle?

  • Glycerate 3-phosphate is reduced to triose phosphate by which two molecules?

  • How many TP molecules are converted into hexose sugars?

  • Which of these molecules produced in the Calvin cycle are NOT converted into useful organic substances?

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