Explain how guard cells control the opening and closing of stomata in a plant leaf.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
When the plant has plenty of water, guard cells take up water by osmosis, becoming turgid and swelling. The uneven, thickened inner wall of the guard cells causes them to curve or bow outward, opening the stoma. When water is scarce or the plant is in dry conditions, guard cells lose water, become flaccid and the stoma closes.
Examiner tips
- Use the word "osmosis" to show understanding of water movement.
- Explain the role of turgidity and the thickened inner wall in changing the guard cell shape.
- Mention both opening and closing conditions to cover all marks.
- Use the term "stoma" (plural stomata) correctly.
Common mistakes
- Forgetting to mention osmosis or turgidity.
- Using the wrong term (e.g., "stomata" instead of "stoma").
- Omitting the thickened inner wall or the direction of the guard cell movement.
Mark scheme (4 marks)
- When the plant has plenty of water, guard cells take up water by osmosis
- Guard cells become turgid / swell up
- The uneven/thickened inner wall causes the guard cells to curve / bow outward, opening the stoma
- When water is scarce / in dry conditions, guard cells lose water / become flaccid and the stoma closes
Key terms in this question
Related
- All WJEC GCSE Biology (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Plant structure and function questions
- Describe how water moves from the soil into a plant and up to the leaves.
- Explain why plants need both xylem and phloem vessels, referring to the substanc…
- Explain how the structure of a xylem vessel is adapted for its function of trans…
- Explain how the structure of a leaf is adapted to allow efficient photosynthesis…