5.1.5

Light-Independent Reaction

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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.

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1) Carbon fixation

  • When CO2 diffuses into the leaves through stomata, it combines with ribulose bisphosphate (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.
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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.
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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.
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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.

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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.
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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.
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Amino acids

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

Jump to other topics

1Biological Molecules

2Cells

3Substance Exchange

4Genetic Information & Variation

5Energy Transfers (A2 only)

6Responding to Change (A2 only)

7Genetics & Ecosystems (A2 only)

8The Control of Gene Expression (A2 only)

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