Explain why the waxy cuticle and the distribution of stomata on a leaf are important for limiting water loss whilst still allowing gas exchange to occur.

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).

Model answer (4 marks)

The waxy cuticle on the upper leaf surface reduces evaporation by forming a hydrophobic barrier, preventing water loss.
Stomata are mainly on the lower (abaxial) surface, away from direct sunlight, which lowers the evaporation rate.
When the stomata open, CO₂ enters and O₂ exits, allowing gas exchange.
In dry or dark conditions the stomata close, reducing water loss when gas exchange is less required.

Examiner tips

  • Use the exact terms ‘waxy cuticle’, ‘abaxial surface’, ‘stomata’, ‘open/close’
  • Show the cause–effect chain: cuticle → less evaporation; lower stomata → less evaporation; opening → gas exchange; closing → water saving

Common mistakes

  • Confusing the cuticle with the epidermis or mistaking the upper surface for the main stomatal surface
  • Failing to mention that stomata can close to reduce water loss

Mark scheme (4 marks)

  1. The waxy cuticle reduces / prevents water loss by evaporation from the upper leaf surface
  2. Stomata are mainly found on the lower (abaxial) surface, away from direct sunlight, which reduces the rate of evaporation / water loss
  3. Stomata can open and close to allow carbon dioxide in and oxygen out (or vice versa) for gas exchange
  4. Stomata close (e.g. in dry / dark conditions) to reduce water loss when gas exchange is less needed

Key terms in this question

waxy cuticle · stomata · gas exchange

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