Describe the role of stomata and the spongy mesophyll layer in gas exchange in a flowering plant.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Stomata are pores in the lower epidermis that allow gases to enter and leave the leaf.
Carbon dioxide diffuses into the leaf down a concentration gradient while oxygen diffuses out during photosynthesis; the reverse occurs during respiration.
The spongy mesophyll contains large air spaces that enable gases to circulate and diffuse throughout the leaf.
The large surface area of spongy mesophyll cells increases the area over which gas exchange can occur.
Carbon dioxide diffuses into the leaf down a concentration gradient while oxygen diffuses out during photosynthesis; the reverse occurs during respiration.
The spongy mesophyll contains large air spaces that enable gases to circulate and diffuse throughout the leaf.
The large surface area of spongy mesophyll cells increases the area over which gas exchange can occur.
Examiner tips
- Use the word "stomata" and "spongy mesophyll" exactly. Show the direction of CO₂ and O₂ diffusion. Mention the role of air spaces and surface area. Keep the answer concise to fit 4 marks.
Common mistakes
- Confusing stomata with guard cells. Forgetting to state the direction of diffusion. Omitting the air‑space function of spongy mesophyll.
Mark scheme (4 marks)
- Stomata are pores (in the lower epidermis) through which gases enter and leave the leaf
- Carbon dioxide diffuses into the leaf (down a concentration gradient) and oxygen diffuses out during photosynthesis / the reverse occurs during respiration
- The spongy mesophyll contains large air spaces that allow gases to circulate / diffuse throughout the leaf
- The large surface area of spongy mesophyll cells increases the area over which gas exchange (diffusion) can occur
Key terms in this question
Related
- All Pearson Edexcel International GCSE Biology (4BI1) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
More Gas exchange: flowering plants questions
- Explain why the waxy cuticle and the distribution of stomata on a leaf are impor…
- Explain why carbon dioxide produced during respiration in the leaf's palisade me…
- Explain why a flowering plant needs both photosynthesis and respiration to occur…
- Explain how the structure of a leaf is adapted to allow efficient gas exchange i…