Explain how the waxy cuticle and the guard cells each help a leaf to reduce water loss whilst still allowing photosynthesis to continue.

Cambridge International IGCSE Biology (0610) — 6.2 Leaf structure · 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 is a thin, waterproof layer covering the epidermis; it prevents water from evaporating from the leaf surface. Because the cuticle is clear, light can still pass through to the palisade mesophyll where photosynthesis occurs.
Guard cells surround each stoma. When water is scarce they become turgid, closing the stoma and stopping water loss through transpiration. Stomata remain open when water is available, allowing CO₂ to enter for photosynthesis.

Examiner tips

  • Mention both cuticle and guard cells; link cuticle to light transmission and guard cells to water balance
  • Use terms ‘waxy cuticle’, ‘transpiration’, ‘stomata’, ‘CO₂’
  • Show cause–effect: cuticle reduces loss but allows light; guard cells close to save water but open to take in CO₂

Common mistakes

  • Forgetting that the cuticle is transparent
  • Confusing the role of guard cells (e.g., saying they always close)
  • Not linking stomatal opening to CO₂ entry

Mark scheme (4 marks)

  1. The waxy cuticle reduces water loss by waterproofing the leaf surface
  2. The waxy cuticle is transparent / clear, so light can still pass through to reach the palisade mesophyll for photosynthesis
  3. Guard cells close the stomata when water supply is limited, reducing water loss through the stomata
  4. Closing the stomata reduces carbon dioxide entry, which can limit photosynthesis, so stomata remain open when water is available to allow carbon dioxide in

Key terms in this question

waxy cuticle · guard cells

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