Explain how the structure of a leaf is adapted to allow efficient gas exchange during photosynthesis.

AQA GCSE Biology (8461) — 4.2.3 Plant tissues, organs and systems · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

1. The leaf is thin and flat, so the distance that gases must diffuse to reach chloroplasts is short.
2. The lower epidermis contains stomata, which open to allow CO₂ to enter and O₂ to leave.
3. Inside the leaf the spongy mesophyll has large air spaces between cells.
4. These air spaces provide a continuous pathway for gases to diffuse rapidly throughout the leaf.

Examiner tips

  • Use the word ‘adapted’ to link structure to function; mention thinness, stomata, spongy mesophyll and air spaces. Keep each point brief and to the point. Use correct terminology: ‘stomata’, ‘spongy mesophyll’, ‘air spaces’.
  • Show the causal chain: structure → function → efficient gas exchange.
  • Use the exact wording from the mark scheme where possible.

Common mistakes

  • Confusing the epidermis with the mesophyll; forgetting to mention the lower epidermis. Failing to state that the leaf is thin and flat. Using vague terms like ‘good for gas exchange’ instead of specific structures.

Mark scheme (4 marks)

  1. Spongy mesophyll contains air spaces / large gaps between cells
  2. Air spaces allow gases (carbon dioxide and oxygen) to diffuse / move through the leaf
  3. Stomata (in the lower epidermis) allow gases to enter and leave the leaf
  4. Leaves are thin / flat so gases only have a short distance to diffuse to reach cells

Key terms in this question

gas exchange

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