The spongy mesophyll layer is an important part of a leaf for gas exchange. Explain how the structure of the spongy mesophyll layer makes gas exchange efficient.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Plants require carbon dioxide for photosynthesis and must also remove oxygen produced during this process. Gas exchange in leaves takes place mainly in the spongy mesophyll layer.
Model answer (5 marks)
The spongy mesophyll cells are loosely packed, creating air spaces between them, which increases the surface area available for gas exchange.
The large surface area allows more CO₂ to enter and O₂ to leave the leaf.
The cells are thin and the diffusion distance is short, so gases diffuse rapidly.
Because CO₂ is continually used in photosynthesis and O₂ is removed, a large concentration gradient is maintained.
The moist cell surfaces enable gases to dissolve in the cell sap before crossing the membrane, further speeding diffusion.
The large surface area allows more CO₂ to enter and O₂ to leave the leaf.
The cells are thin and the diffusion distance is short, so gases diffuse rapidly.
Because CO₂ is continually used in photosynthesis and O₂ is removed, a large concentration gradient is maintained.
The moist cell surfaces enable gases to dissolve in the cell sap before crossing the membrane, further speeding diffusion.
Examiner tips
- Use the exact terms: ‘loosely packed’, ‘air spaces’, ‘surface area’, ‘short diffusion distance’, ‘concentration gradient’, ‘moist cell surfaces’.
- Show the causal chain: structure → increased area & short distance → faster diffusion → efficient gas exchange.
Common mistakes
- Confusing the role of stomata with spongy mesophyll structure.
- Omitting the importance of a concentration gradient or moist surfaces.
Mark scheme (5 marks)
- The spongy mesophyll cells are loosely packed, creating air spaces between them
- The air spaces increase the surface area available for gas exchange
- Gases diffuse faster because the diffusion distance is short / cells are thin
- A large concentration gradient is maintained because gases are used up or removed
- The moist cell surfaces allow gases to dissolve before diffusing across the cell membrane
Key terms in this question
Related
- All Eduqas A-Level Biology revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Adaptations for gas exchange questions
- A leaf is adapted for efficient gas exchange. Describe the features of a leaf th…
- The stomata in a leaf open and close to control gas exchange. Explain how the st…
- Fish gills are adapted for efficient gas exchange between water and the blood. D…
- Explain how the alveoli in the lungs are adapted for efficient gas exchange.