OCR · A-Level · Biology

    Course version: H420 · Version from Sept 2025

    Photosynthesis: (g) (i) factors affecting photosynthesis

    Photosynthesis rate is limited by whichever factor is in shortest supply.

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

    • Point 8 of 9
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Photosynthesis rate is limited by whichever factor is in shortest supply. Light intensity, carbon dioxide concentration and temperature each act on a different stage, so raising one only increases the rate until another becomes limiting. Light drives the light-dependent stage, producing ATP and reduced NADP; carbon dioxide supply feeds the light-independent stage via rubisco-catalysed carbon fixation; temperature affects enzyme activity, including rubisco, and so changes rate with a distinct optimum before denaturation reduces it. A graph of rate against one factor, with the others held constant, plateaus where a second factor limits. Learners should read such graphs, identify the limiting factor at a given point, and predict the effect of changing conditions.

    What earns the marks

    • Light intensity affects the light-dependent stage; increasing it raises the rate until another factor limits, giving a plateau.
    • Carbon dioxide concentration limits the light-independent stage; raising it increases the rate up to a plateau set by another factor.
    • Temperature affects enzyme-controlled reactions such as carbon fixation; rate rises to an optimum then falls as enzymes denature.
    • The limiting factor is the one in shortest supply; changing a non-limiting factor has little or no effect on rate.
    • Reading a rate-versus-factor graph: identify the plateau and state which other factor is then limiting.

    Common mistakes and how to put them right

    • Assuming more light always increases rate indefinitely.

      Put it right: Once another factor limits, extra light gives no further increase.

    • Treating temperature as affecting only the light-dependent stage.

      Put it right: Temperature chiefly affects enzyme-controlled reactions including the light-independent stage.

    • Believing any factor raised will raise the rate.

      Put it right: Only the limiting factor changes the rate.

    Exam tips

    • State which stage each factor acts on, not just that it 'affects photosynthesis'.
    • When interpreting a graph, name the limiting factor at the specific point asked about.
    • Use 'limiting factor' precisely: the factor in shortest supply relative to need.

    Try the lesson

    The start of the Photosynthesis: (g) (i) factors affecting photosynthesis lesson, made from checked teaching notes for this specification point.

    Have a go first

    A student graphs the rate of photosynthesis of a plant against light intensity. At very high light intensities the graph levels off. Which factor is now limiting the rate?

    Listen

    Photosynthesis rate is limited by whichever factor is in shortest supply. Light intensity, carbon dioxide concentration and temperature each act on a different stage, so raising one only increases the rate until another becomes limiting. Light drives the light-dependent stage, producing ATP and reduced NADP; carbon dioxide supply feeds the light-independent stage via rubisco-catalysed carbon fixation; temperature affects enzyme activity, including rubisco, and so changes rate with a distinct optimum before denaturation reduces it. A graph of rate against one factor, with the others held constant, plateaus where a second factor limits. Learners should read such graphs, identify the limiting factor at a given point, and predict the effect of changing conditions. A strong answer earns marks for these points. Light intensity affects the light-dependent stage; increasing it raises the rate until another factor limits, giving a plateau. Carbon dioxide concentration limits the light-independent stage; raising it increases the rate up to a plateau set by another factor. Temperature affects enzyme-controlled reactions such as carbon fixation; rate rises to an optimum then falls as enzymes denature. The limiting factor is the one in shortest supply; changing a non-limiting factor has little or no effect on rate. A common slip: Assuming more light always increases rate indefinitely. Instead, once another factor limits, extra light gives no further increase. Exam tip: State which stage each factor acts on, not just that it 'affects photosynthesis'.

    See it

    A top-down flowchart. Light intensity leads to the light-dependent stage, which produces ATP and reduced NADP, which feeds into the rate of photosynthesis. Carbon dioxide concentration leads to the light-independent stage, then to rubisco-catalysed carbon fixation, which also feeds into the rate of photosynthesis. Temperature leads to enzyme activity, including rubisco, which also feeds into the rate of photosynthesis. The rate of photosynthesis leads to the limiting factor, defined as the factor in shortest supply, which leads to the point that raising a non-limiting factor has little or no effect.
    Shows how light intensity, carbon dioxide concentration and temperature each act on a different stage to affect the rate of photosynthesis, and that the factor in shortest supply is the limiting factor.

    Key words

    Say what each word means before you turn the card.

    Limiting factors

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

    Light-dependent and light-independent stages

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

    Enzyme activity and temperature

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

    Graph interpretation

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

    See every definition
    Limiting factors
    The factor in shortest supply relative to need; changing a non-limiting factor has little or no effect on rate.
    Light-dependent and light-independent stages
    Light drives the light-dependent stage, producing ATP and reduced NADP, while carbon dioxide supply feeds the light-independent stage via rubisco-catalysed carbon fixation.
    Enzyme activity and temperature
    Temperature affects enzyme-controlled reactions such as carbon fixation, so rate rises to an optimum then falls as enzymes denature.
    Graph interpretation
    Reading a rate-versus-factor graph to identify the plateau and state which other factor is then limiting.

    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 top-down flowchart. Light intensity leads to the light-dependent stage, which produces ATP and reduced NADP, which feeds into the rate of photosynthesis. Carbon dioxide concentration leads to the light-independent stage, then to rubisco-catalysed carbon fixation, which also feeds into the rate of photosynthesis. Temperature leads to enzyme activity, including rubisco, which also feeds into the rate of photosynthesis. The rate of photosynthesis leads to the limiting factor, defined as the factor in shortest supply, which leads to the point that raising a non-limiting factor has little or no effect.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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