Explain why water vapour is considered a waste product of metabolism in a flowering plant and describe how it is removed from the plant.

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)

Water is produced as a by‑product of aerobic respiration in the mitochondria of plant cells. The excess water cannot be stored and would damage cells if it accumulated, so it must be removed. The water vapour diffuses out of the leaf through the stomata, moving down a concentration gradient from inside the leaf to the atmosphere. It can also escape through lenticels in the stems or, to a lesser extent, through the cuticle.

Examiner tips

  • Use the term ‘by‑product of aerobic respiration’ to gain the first point. Show that excess water cannot be stored and would be harmful. Explain diffusion through stomata and mention lenticels or cuticle as alternatives.
  • Use clear, exam‑style language and keep the answer concise to fit the 4 marks.

Common mistakes

  • Confusing water as a useful product rather than a waste. Forgetting that water must be removed because it cannot be stored. Omitting the diffusion through stomata or the role of lenticels.

Mark scheme (4 marks)

  1. Water is produced as a waste product during aerobic respiration (in the mitochondria) / water is a by-product of metabolic reactions
  2. Excess water (vapour) must be removed / it is excreted because it cannot be stored / it would be harmful if it accumulated
  3. Water vapour diffuses out through the stomata (in the leaves) / moves down a concentration gradient from inside the leaf to the atmosphere
  4. Water vapour can also be lost through the lenticel(s) in stems / through the cuticle (though the waxy cuticle reduces this loss)

Related

More Excretion: flowering plants questions

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