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

Exam code: 7402

Lára Marie McIvor

Written by: Lára Marie McIvor

Reviewed by: Naomi Holyoak

Updated on

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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:

    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

Diagram of the Calvin cycle showing the conversion of carbon dioxide to glucose, involving RuBP, ATP, and NADPH, with arrows indicating the process flow.
The Calvin cycle is a series of reactions that allow the continuous regeneration of RuBP as well as the production of useful organic compounds for the plant
  • 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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Lára Marie McIvor

Author: Lára Marie McIvor

Expertise: Biology, Psychology & Sociology Subject Lead

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.

Naomi Holyoak

Reviewer: Naomi Holyoak

Expertise: Biology Content Creator

Naomi graduated from the University of Oxford with a degree in Biological Sciences. She has 8 years of classroom experience teaching Key Stage 3 up to A-Level biology, and is currently a tutor and A-Level examiner. Naomi especially enjoys creating resources that enable students to build a solid understanding of subject content, while also connecting their knowledge with biology’s exciting, real-world applications.