Describe how the rate of photosynthesis in a plant would change if carbon dioxide concentration in the air gradually increased. Explain why this change occurs.

Eduqas GCSE Biology — 1.5.2 Photosynthesis · Describe · 4 marks · View as Markdown

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

A plant is growing in a sealed greenhouse. Over several hours, the concentration of carbon dioxide inside the greenhouse slowly rises.

Model answer (4 marks)

The rate of photosynthesis will increase as the concentration of CO₂ rises, because CO₂ is a reactant in the light‑independent reactions. With more CO₂ available, more carbon‑fixing reactions can occur, producing more glucose. However, the rate will eventually level off when another factor (e.g. light intensity or temperature) becomes limiting.

Examiner tips

  • Use the word ‘increase’ and explain CO₂ is a reactant; mention the limiting factor at the end.
  • Show the cause–effect chain: higher CO₂ → more reactions → more glucose.
  • Keep the answer concise and use the exact terminology from the mark scheme.

Common mistakes

  • Saying the rate will keep rising indefinitely; ignoring the limiting factor.
  • Using vague terms like ‘more energy’ instead of ‘more CO₂ as a reactant’.

Mark scheme (4 marks)

  1. The rate of photosynthesis increases (as carbon dioxide concentration rises)
  2. Carbon dioxide is a reactant / raw material used in photosynthesis
  3. More carbon dioxide available means more reactions can take place / more glucose can be produced
  4. The rate will eventually level off / stop increasing because another factor becomes limiting (e.g. light intensity or temperature)

Key terms in this question

photosynthesis · carbon dioxide · rate of photosynthesis

Related

More Photosynthesis questions

▶ Try answering this question with AI marking (free) →