Explain how the skin of a frog functions as a gas exchange surface.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Frogs are amphibians that can exchange gases across their skin as well as through their lungs. Cutaneous respiration accounts for a significant proportion of total gas exchange, particularly when the frog is at rest or submerged in water.
Model answer (4 marks)
The skin of a frog is very thin, with a thin epithelium that shortens the diffusion distance for gases.
The skin is kept moist by a mucus layer, which allows oxygen and carbon dioxide to dissolve before diffusing across the membrane.
A dense network of capillaries lies just beneath the skin surface, creating a steep concentration gradient that drives diffusion.
The skin has a large surface area relative to the frog’s body volume, providing enough area for cutaneous respiration to meet metabolic demands.
The skin is kept moist by a mucus layer, which allows oxygen and carbon dioxide to dissolve before diffusing across the membrane.
A dense network of capillaries lies just beneath the skin surface, creating a steep concentration gradient that drives diffusion.
The skin has a large surface area relative to the frog’s body volume, providing enough area for cutaneous respiration to meet metabolic demands.
Examiner tips
- Use the exact terms: thin epithelium, mucus layer, capillaries, large surface area.
- Show the causal link: thin skin → short distance; mucus → dissolution; capillaries → gradient; area → capacity.
- Keep each point brief and separate for clarity.
Common mistakes
- Writing that the skin is thick or ignoring the mucus layer.
- Failing to mention the capillary network or the importance of surface area.
Mark scheme (4 marks)
- The skin is thin / has a thin epithelium, reducing the diffusion distance for gases
- The skin must remain moist / covered in a mucus layer so that gases dissolve before diffusing across the membrane
- A dense network of capillaries lies close to the skin surface, maintaining a steep concentration gradient for diffusion
- The large surface area of the skin relative to body volume provides a sufficiently large area for gas exchange to meet metabolic demands
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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