A leaf is adapted for efficient gas exchange. Describe the adaptations of a leaf that allow efficient gas exchange between the leaf and the surrounding air.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Leaves are the main organs of gas exchange in plants. Carbon dioxide must enter the leaf for photosynthesis, and oxygen and water vapour must be able to leave.
Model answer (5 marks)
Stomata are situated on the lower epidermis, allowing CO₂ to enter and O₂ and H₂O vapour to leave.
Guard cells surrounding each stoma control its opening and closing, regulating the rate of gas exchange.
The spongy mesophyll contains large intercellular air spaces, which provide a rapid pathway for gases to diffuse through the leaf.
A large leaf surface area and a high density of stomata increase the total amount of gas that can diffuse per unit time.
The waxy cuticle prevents excessive water loss, yet it does not block gas diffusion through the stomata; cells remain moist so gases dissolve in the cytoplasm before crossing the plasma membrane.
Guard cells surrounding each stoma control its opening and closing, regulating the rate of gas exchange.
The spongy mesophyll contains large intercellular air spaces, which provide a rapid pathway for gases to diffuse through the leaf.
A large leaf surface area and a high density of stomata increase the total amount of gas that can diffuse per unit time.
The waxy cuticle prevents excessive water loss, yet it does not block gas diffusion through the stomata; cells remain moist so gases dissolve in the cytoplasm before crossing the plasma membrane.
Examiner tips
- Mention stomata on the lower surface first; use "guard cells" to show control. Highlight spongy mesophyll air spaces. Include leaf surface area and stomatal density. Explain cuticle’s role in water loss but not gas exclusion.
Common mistakes
- Stating stomata are on the upper surface only. Forgetting to mention guard cells. Ignoring the role of the spongy mesophyll or cuticle.
Mark scheme (5 marks)
- Stomata are present on the lower epidermis / lower surface of the leaf, which allow gases to diffuse in and out
- Guard cells control the opening and closing of stomata, regulating gas exchange
- The spongy mesophyll layer has air spaces which allow fast diffusion of gases through the leaf
- The large surface area of the leaf / many stomata increases the rate of diffusion / amount of gas exchanged
- The waxy cuticle reduces water loss but does not prevent gas exchange through the stomata / cells are moist which allows gases to dissolve before diffusing across the membrane
Related
- All WJEC A-Level Biology (Wales) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
More Adaptations for gas exchange questions
- Fish gills are adapted for efficient gas exchange. Describe the adaptations of f…
- The spongy mesophyll and the alveoli in the lungs are both gas exchange surfaces…
- The spongy mesophyll layer is an important part of a leaf. Explain how the featu…
- Explain how the alveoli in the lungs are adapted for efficient gas exchange.