The Light-Independent Reaction (AQA A Level Biology): Revision Note

The light-independent reactions

• The products of the light-dependent reactions that pass to the light-independent reactions are:

  • ATP

  • reduced NADP (NADPH)

• ATP and reduced NADP are used during the light-independent reactions to produce simple sugar molecules, which can then be converted into complex organic molecules

  • The energy from ATP is released during ATP hydrolysis

• The need for ATP and reduced NADP from the light-dependent reactions means that the light independent reactions are not truly independent of light; they cannot continue indefinitely in darkness, as these inputs will run out

The Calvin cycle

• The reactions of the light-independent stage are known collectively as the Calvin cycle

• The Calvin cycle involves:

  1. fixation of carbon dioxide

     • Carbon dioxide combines with a five-carbon (5C) sugar known as ribulose bisphosphate (RuBP) in a reaction catalysed by the enzyme rubisco (ribulose bisphosphate carboxylase)

     • The resulting six-carbon (6C) compound is unstable and splits to produce two molecules of a three-carbon (3C) compound known as glycerate 3-phosphate (GP)

  2. the reduction of glycerate 3-phosphate to triose phosphate

     • ATP and reduced NADP are used to reduce GP to triose phosphate

  3. the regeneration of ribulose bisphosphate

     • ATP is used to convert five of the six carbons present in the two triose phosphate molecules back into RuBP

  4. the production of useful organic substances

     • Remaining triose phosphate molecules are used to produce useful organic molecules needed by the plant, e.g. starch, cellulose, lipids and proteins

• Triose phosphate can be converted into useful organic substances as follows:

  • Triose phosphate can condense to form hexose phosphates (6C), which can be used to produce:

    • starch

    • sucrose

    • cellulose

  • Triose phosphate can be converted to glycerol and GP to fatty acids, which join to form lipids, e.g. for cell membranes

  • Triose phosphate can be used in the production of amino acids for protein synthesis