Explain why the rate of photosynthesis in a plant decreases when the concentration of carbon dioxide in the air falls below a certain level.

AQA GCSE Biology (8461) — 4.4.1 Photosynthesis · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Carbon dioxide is a reactant in photosynthesis.
When the CO₂ concentration falls below a certain level it becomes a limiting factor.
The plant then cannot take in enough CO₂ to keep the current rate of the reaction.
If the CO₂ concentration were increased again, the rate of photosynthesis would rise until another factor became limiting.

Examiner tips

Use the word ‘limiting factor’ to show understanding of the concept.
Explain that CO₂ is a reactant and that its low level limits the reaction rate.
Mention that increasing CO₂ would raise the rate until another factor limits it.
Keep the answer concise – 4 points are enough for full marks.

Common mistakes

Confusing CO₂ with O₂ or water as the limiting factor.
Failing to state that CO₂ is a reactant in the light‑independent reactions.
Giving a generic explanation without linking CO₂ concentration to the reaction rate.

Mark scheme (4 marks)

Carbon dioxide is a reactant in / needed for photosynthesis
Carbon dioxide acts as a limiting factor (when in low concentration)
There is not enough carbon dioxide to maintain the current rate of reaction / the plant cannot photosynthesise as fast
Increasing carbon dioxide concentration (above this level) would increase the rate of photosynthesis / another factor then becomes limiting

Key terms in this question

photosynthesis · carbon dioxide