The spongy mesophyll and the alveoli in the lungs are both gas exchange surfaces. Explain how the features of the spongy mesophyll layer make it an efficient gas exchange surface.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Plants exchange gases through internal leaf structures. Carbon dioxide must reach the palisade cells for photosynthesis, and oxygen must be able to leave the leaf. The spongy mesophyll layer plays a key role in this process.
Model answer (5 marks)
The spongy mesophyll contains loosely packed cells that create large air spaces.
1. The air spaces give a high surface area for gas exchange.
2. The large surface area maintains a steep concentration gradient, so CO₂ diffuses into the cells and O₂ diffuses out.
3. The cell walls are moist, allowing CO₂ and O₂ to dissolve in the intercellular fluid before crossing the membrane.
4. The cells are thin and the diffusion distance is short, so gases move rapidly across the membrane.
5. Together these features make the spongy mesophyll an efficient gas‑exchange surface.
1. The air spaces give a high surface area for gas exchange.
2. The large surface area maintains a steep concentration gradient, so CO₂ diffuses into the cells and O₂ diffuses out.
3. The cell walls are moist, allowing CO₂ and O₂ to dissolve in the intercellular fluid before crossing the membrane.
4. The cells are thin and the diffusion distance is short, so gases move rapidly across the membrane.
5. Together these features make the spongy mesophyll an efficient gas‑exchange surface.
Examiner tips
- Use the word ‘air spaces’ and ‘high surface area’ to hit the first two points. Show the link between surface area, concentration gradient and diffusion speed. Mention moist cell walls and thin cells to cover the last two points.
- Include all five points in a logical order – start with structure, then surface area, gradient, moist walls, thin cells.
Mark scheme (5 marks)
- The spongy mesophyll has air spaces between the loosely packed cells
- Air spaces increase the surface area available for gas exchange
- A large concentration gradient is maintained / gases are used up or produced in photosynthesis or respiration
- The cell walls / surfaces of the spongy mesophyll cells are moist, allowing gases to dissolve before diffusing across the membrane
- The cells of the spongy mesophyll are thin / have a short diffusion distance, so gases diffuse quickly across the membrane
Key terms in this question
Related
- All WJEC A-Level Biology (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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