Explain why a flowering plant needs both photosynthesis and respiration to occur simultaneously during the day, with reference to the gases involved in gas exchange.

Pearson Edexcel International GCSE Biology (4BI1) — 2.8 Gas exchange: flowering plants · Explain · 4 marks · View as Markdown

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

A flowering plant is kept in bright light for several hours during the day. Gas exchange occurs through the stomata on the leaf surface.

Model answer (4 marks)

1. Photosynthesis takes in CO₂ from the air.
2. It releases O₂, which diffuses out through the stomata.
3. Respiration requires O₂ to oxidise glucose.
4. Respiration releases CO₂, which can be reused in photosynthesis, giving a net uptake of CO₂ in bright light.

Examiner tips

  • Use the exact terms CO₂, O₂ and stomata; link each gas to the correct process.
  • Show the cycle: CO₂ in → photosynthesis → O₂ out; O₂ in → respiration → CO₂ out.
  • Mention that the CO₂ produced by respiration is available for photosynthesis, giving a net CO₂ uptake.

Common mistakes

  • Confusing the direction of gas flow (e.g., saying O₂ is taken in by photosynthesis).
  • Omitting the role of stomata or the net CO₂ uptake in bright light.

Mark scheme (4 marks)

  1. Photosynthesis uses / takes in carbon dioxide (from the air / surroundings)
  2. Photosynthesis releases / produces oxygen (which diffuses out through stomata)
  3. Respiration uses / requires oxygen (to release energy from glucose)
  4. Respiration produces / releases carbon dioxide, which can be used directly in photosynthesis / so gas exchange involves a net uptake of carbon dioxide in bright light

Key terms in this question

photosynthesis · respiration · gas exchange

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