OCR · A-Level · Biology

    Course version: H420 · Version from Sept 2025

    (e) the fixation of carbon dioxide and the light-independent stage of…

    Full specification wording

    Photosynthesis: (e) the fixation of carbon dioxide and the light-independent stage of photosynthesis

    The light-independent stage occurs in the stroma and does not need light directly, but depends on ATP and reduced NADP from the light-dependent stage.

    Checked against the OCR H420 specification (opens in a new tab) on .

    • Point 6 of 9
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    The light-independent stage occurs in the stroma and does not need light directly, but depends on ATP and reduced NADP from the light-dependent stage. Carbon dioxide is fixed when it joins the five-carbon acceptor ribulose bisphosphate (RuBP), catalysed by rubisco, forming two molecules of the three-carbon glycerate 3-phosphate (GP). GP is reduced to triose phosphate (TP) using ATP and reduced NADP. Most TP regenerates RuBP, consuming further ATP, so the cycle continues. A useful check: for each CO₂ fixed, one carbon enters the cycle, and the cycle must turn six times to build one hexose. Track the carbon atoms and the ATP and reduced NADP consumed at each step.

    What earns the marks

    • Carbon dioxide is fixed by combining with the five-carbon compound ribulose bisphosphate (RuBP), a reaction catalysed by the enzyme rubisco.
    • The unstable six-carbon product immediately splits into two molecules of the three-carbon glycerate 3-phosphate (GP).
    • GP is reduced to triose phosphate (TP) using ATP and reduced NADP supplied by the light-dependent stage.
    • Most TP is used to regenerate RuBP, which requires further ATP, so the cycle is self-sustaining.
    • The stage occurs in the stroma and does not require light directly, but stops without the products of the light-dependent reactions.

    Common mistakes and how to put them right

    • Saying the light-independent stage needs light directly.

      Put it right by explaining it depends on ATP and reduced NADP from the light-dependent stage, not on light itself.

    • Naming the CO₂ acceptor as a three-carbon compound.

      Put it right by stating the acceptor is the five-carbon RuBP.

    • Forgetting that ATP is used both in reducing GP and in regenerating RuBP.

      Put it right by listing both ATP-consuming steps.

    Exam tips

    • Learn the carbon-atom count at each step (5-carbon RuBP, 3-carbon GP and TP) so you can trace the cycle accurately.
    • Link each step to the enzyme or coenzyme involved, such as rubisco for fixation and reduced NADP for reduction.
    • Practise explaining why the cycle stops in darkness even though it does not use light directly.

    Try the lesson

    The start of the Photosynthesis: (e) the fixation of carbon dioxide and the light-independent stage of photosynthesis lesson, made from checked teaching notes for this specification point.

    Have a go first

    In the light-independent stage of photosynthesis, which enzyme catalyses the fixation of carbon dioxide onto ribulose bisphosphate (RuBP)?

    Listen

    The light-independent stage occurs in the stroma and does not need light directly, but depends on ATP and reduced NADP from the light-dependent stage. Carbon dioxide is fixed when it joins the five-carbon acceptor ribulose bisphosphate (RuBP), catalysed by rubisco, forming two molecules of the three-carbon glycerate 3-phosphate (GP). GP is reduced to triose phosphate (TP) using ATP and reduced NADP. Most TP regenerates RuBP, consuming further ATP, so the cycle continues. A useful check: for each CO₂ fixed, one carbon enters the cycle, and the cycle must turn six times to build one hexose. Track the carbon atoms and the ATP and reduced NADP consumed at each step. A strong answer earns marks for these points. Carbon dioxide is fixed by combining with the five-carbon compound ribulose bisphosphate (RuBP), a reaction catalysed by the enzyme rubisco. The unstable six-carbon product immediately splits into two molecules of the three-carbon glycerate 3-phosphate (GP). GP is reduced to triose phosphate (TP) using ATP and reduced NADP supplied by the light-dependent stage. Most TP is used to regenerate RuBP, which requires further ATP, so the cycle is self-sustaining. A common slip: Saying the light-independent stage needs light directly. Instead, explaining it depends on ATP and reduced NADP from the light-dependent stage, not on light itself. Exam tip: Learn the carbon-atom count at each step (5-carbon RuBP, 3-carbon GP and TP) so you can trace the cycle accurately.

    See it

    A flowchart of the light-independent stage. Carbon dioxide fixes onto the five-carbon compound RuBP, catalysed by rubisco, forming an unstable six-carbon product that splits into two molecules of the three-carbon GP. GP is reduced to TP using ATP and reduced NADP. Most TP is used to regenerate RuBP, which requires further ATP, and an arrow loops back from regeneration to the fixation step so the cycle continues. A separate node shows the light-dependent stage supplying ATP and reduced NADP, with arrows pointing to both the reduction of GP to TP and the regeneration of RuBP.
    Shows the light-independent stage as a cycle: carbon dioxide fixation onto RuBP, splitting into GP, reduction to TP using ATP and reduced NADP, and regeneration of RuBP.

    Key words

    Say what each word means before you turn the card.

    Carbon fixation by rubisco

    Try answering aloud or in your head before turning the card.

    Reduction of GP to TP

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    Regeneration of RuBP

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    Dependence on the light-dependent stage

    Try answering aloud or in your head before turning the card.

    See every definition
    Carbon fixation by rubisco
    Carbon dioxide joins the five-carbon acceptor ribulose bisphosphate (RuBP), a reaction catalysed by the enzyme rubisco.
    Reduction of GP to TP
    Glycerate 3-phosphate (GP) is reduced to triose phosphate (TP) using ATP and reduced NADP supplied by the light-dependent stage.
    Regeneration of RuBP
    Most TP is used to regenerate RuBP, which requires further ATP, so the cycle is self-sustaining.
    Dependence on the light-dependent stage
    The stage occurs in the stroma and does not require light directly, but stops without the products of the light-dependent reactions.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A flowchart of the light-independent stage. Carbon dioxide fixes onto the five-carbon compound RuBP, catalysed by rubisco, forming an unstable six-carbon product that splits into two molecules of the three-carbon GP. GP is reduced to TP using ATP and reduced NADP. Most TP is used to regenerate RuBP, which requires further ATP, and an arrow loops back from regeneration to the fixation step so the cycle continues. A separate node shows the light-dependent stage supplying ATP and reduced NADP, with arrows pointing to both the reduction of GP to TP and the regeneration of RuBP.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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