A student investigates gas exchange in a plant. They place a leafy shoot in bright light and then move it into complete darkness. Describe how the movement of carbon dioxide and oxygen through the stomata changes when the plant is moved from bright light into complete darkness.

AQA A-Level Biology (7402) — 3.3.2 Gas Exchange · Describe · 5 marks · View as Markdown

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

Model answer (5 marks)

1. In bright light the leaf carries out photosynthesis.
2. CO₂ diffuses into the leaf through the stomata to supply the Calvin cycle.
3. O₂ produced by photosynthesis diffuses out of the leaf through the stomata.
4. In complete darkness photosynthesis stops, so the leaf only performs respiration.
5. CO₂ produced by respiration diffuses out of the leaf, while O₂ diffuses in through the stomata.

Examiner tips

  • Use the word ‘diffuses’ to show movement. Mention photosynthesis in light and respiration in darkness. Keep each point short and to the point.
  • common_mistakes
  • :
  • Confusing the direction of CO₂ and O₂ movement. Forgetting that photosynthesis stops in darkness. Using vague terms like ‘moves’ instead of ‘diffuses’.

Mark scheme (5 marks)

  1. In bright light, the plant carries out photosynthesis
  2. In bright light, carbon dioxide moves into the leaf (through stomata) for photosynthesis
  3. In bright light, oxygen moves out of the leaf (through stomata) as a product of photosynthesis
  4. In darkness, photosynthesis stops so the plant only respires
  5. In darkness, carbon dioxide moves out of the leaf and oxygen moves into the leaf (through stomata) for respiration

Key terms in this question

stomata · gas exchange

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