Describe how the rate of photosynthesis in a plant is affected when the concentration of carbon dioxide in the air increases from a very low level to a high level.

WJEC GCSE Biology (Wales) — 1.5.2 Photosynthesis · Describe · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

At very low CO₂ concentrations the rate of photosynthesis is low because CO₂ is a raw material for the Calvin cycle.
As the CO₂ concentration rises, the rate of photosynthesis increases because more CO₂ is available for fixation.
When CO₂ concentration becomes high the rate levels off or plateaus, as another factor (e.g. light intensity or enzyme activity) becomes limiting.

Examiner tips

  • Use the word ‘rate’ and ‘CO₂’ explicitly; show the change from low to high. Mention CO₂ as a reactant. Explain the plateau by citing a limiting factor.
  • Include the sequence: low → increase → plateau.

Mark scheme (4 marks)

  1. At low carbon dioxide concentrations, the rate of photosynthesis is low
  2. As carbon dioxide concentration increases, the rate of photosynthesis increases
  3. Carbon dioxide is a raw material / reactant used in photosynthesis
  4. At high carbon dioxide concentrations the rate levels off / plateaus because another factor becomes limiting

Key terms in this question

photosynthesis · carbon dioxide

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