The Light-Independent Reaction (AQA A Level Biology): Revision Note
Exam code: 7402
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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:
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)
the reduction of glycerate 3-phosphate to triose phosphate
ATP and reduced NADP are used to reduce GP to triose phosphate
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
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
Examiner Tips and Tricks
Note that while the specification uses abbreviations for GP and RuBP, it does not use an abbreviation for triose phosphate, which is often abbreviated in textbooks and notes to TP; be sure to always write triose phosphate in full the first time you use the term in an exam.
Be careful not to confuse the Calvin cycle (photosynthesis) with the Krebs cycle (respiration)!
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