Explain the roles of stomata and guard cells in controlling water loss from a plant leaf.

Edexcel GCSE Biology (1BI0) — 6.2 Transport in 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)

Stomata are small pores on the leaf surface that allow water vapour to escape by diffusion during transpiration.
Guard cells flank each stoma and control its opening.
When guard cells absorb water they become turgid, bending the cell walls and opening the stoma, which increases water loss.
When guard cells lose water they become flaccid, the cell walls straighten and the stoma closes, reducing water loss.

Examiner tips

Use the word ‘stomata’ and ‘guard cells’ explicitly. Explain the change in turgor pressure. Show the cause–effect link between water loss and stomatal state.
Include both opening and closing mechanisms to cover all marks.

Common mistakes

Confusing stomata with guard cells. Forgetting that turgor change is the key mechanism. Using vague terms like ‘open’ or ‘close’ without linking to water movement.

Mark scheme (4 marks)

Stomata are small holes/pores on the surface of a leaf through which water vapour can be lost by diffusion/evaporation (transpiration).
Guard cells control whether the stomata are open or closed.
When guard cells take in water they become turgid, causing the stomata to open, increasing water loss.
When guard cells lose water they become flaccid, causing the stomata to close, reducing water loss.

Key terms in this question

stomata · guard cells