Explain why carbon dioxide is described as an excretory product in a flowering plant, and describe how it leaves the plant during the hours of darkness.

Pearson Edexcel International GCSE Biology (4BI1) — 2.12 Excretion: flowering plants · 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 waste product of aerobic respiration, produced when cells metabolise glucose. Excretion is the removal of such waste products from the plant, and CO₂ must be removed because it can be harmful if it accumulates. In the dark, CO₂ produced in mesophyll cells diffuses down its concentration gradient out of the cells, then through the open stomata and into the surrounding air.

Examiner tips

  • Use the word ‘waste product’ and link CO₂ to respiration; explain removal is necessary.
  • Show the diffusion pathway: mesophyll cells → stomata → atmosphere.
  • Mention that stomata are open during darkness, allowing CO₂ to leave.

Common mistakes

  • Confusing CO₂ with water or mistaking it for a nutrient; not stating it is a waste product.
  • Failing to mention diffusion or the role of stomata; saying CO₂ leaves by transpiration.
  • Using vague terms like ‘leaves the plant’ without explaining the mechanism.

Mark scheme (4 marks)

  1. Carbon dioxide is a waste product of (aerobic) respiration / produced by metabolism
  2. Excretion is the removal of waste products of metabolism from the body / carbon dioxide must be removed as it is a waste product that could be harmful if it accumulated
  3. Carbon dioxide moves out of leaf cells / mesophyll cells by diffusion (down a concentration gradient)
  4. Carbon dioxide diffuses out of the leaf through open stomata (into the surrounding air / atmosphere)

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