A leaf is a gas exchange surface in plants. Explain how the structure of a leaf is adapted to allow efficient gas exchange.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Plants need to exchange carbon dioxide and oxygen with their surroundings during photosynthesis and respiration. The leaf is the main gas exchange surface in a plant.
Model answer (5 marks)
Stomata are small pores in the epidermis that allow gases to enter and leave the leaf.
Guard cells surround each stoma and control its opening and closing, regulating the amount of gas that can diffuse.
The leaf has a large surface area, which increases the rate of diffusion of gases.
The leaf is thin, so gases only have a short distance to diffuse to reach the cells.
Gases (CO₂ and O₂) move by diffusion down a concentration gradient from high to low concentration.
Guard cells surround each stoma and control its opening and closing, regulating the amount of gas that can diffuse.
The leaf has a large surface area, which increases the rate of diffusion of gases.
The leaf is thin, so gases only have a short distance to diffuse to reach the cells.
Gases (CO₂ and O₂) move by diffusion down a concentration gradient from high to low concentration.
Examiner tips
- Use the exact terms ‘stomata’, ‘guard cells’, ‘large surface area’, ‘thin leaf’, and ‘diffusion down a concentration gradient’.
- Show the causal link: structure → function → efficient gas exchange.
- Keep the answer concise, matching the five points.
Mark scheme (5 marks)
- Stomata are small pores in the epidermis that allow gases to enter and leave the leaf
- Guard cells control the opening and closing of the stomata, regulating gas exchange
- The leaf has a large surface area which increases the rate of diffusion of gases
- The leaf is thin so gases only have a short distance to diffuse to reach the cells
- Gases (carbon dioxide and oxygen) move by diffusion down a concentration gradient from high to low concentration
Related
- All OCR A-Level Biology B — Advancing Biology (H422) revision notes →
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