Explain why a single-celled organism such as Amoeba can rely solely on diffusion to meet its respiratory needs, whereas a mammal cannot.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Amoeba is a unicellular organism approximately 0.5 mm in diameter. Mammals are multicellular organisms with body volumes many orders of magnitude greater, composed of metabolically active tissues throughout.
Model answer (4 marks)
Amoeba has a large surface area to volume ratio, so the surface area available for diffusion is sufficient relative to the volume of respiring cytoplasm.
Mammals have a small surface area to volume ratio, so the body surface alone cannot supply enough oxygen to all cells.
The diffusion distance from the exchange surface to cells deep within a mammal is too large for diffusion alone to supply oxygen quickly enough.
Therefore mammals require specialised exchange surfaces and a mass‑transport system to deliver oxygen efficiently to all cells.
Mammals have a small surface area to volume ratio, so the body surface alone cannot supply enough oxygen to all cells.
The diffusion distance from the exchange surface to cells deep within a mammal is too large for diffusion alone to supply oxygen quickly enough.
Therefore mammals require specialised exchange surfaces and a mass‑transport system to deliver oxygen efficiently to all cells.
Examiner tips
- Use the term ‘surface area to volume ratio’ explicitly. Explain the effect of size on diffusion distance. Mention specialised exchange surfaces and mass transport.
- Show the logical progression: ratio → distance → need for transport.
Common mistakes
- Confusing surface area with volume. Failing to mention diffusion distance. Omitting the need for specialised exchange surfaces.
Mark scheme (4 marks)
- Amoeba has a large surface area to volume ratio, so the surface area available for diffusion is sufficient relative to the volume of respiring cytoplasm
- In a mammal the surface area to volume ratio is small/low, so the body surface alone cannot supply sufficient oxygen to all cells
- The diffusion distance from the exchange surface to respiring cells deep within a mammal is too large for diffusion alone to supply oxygen quickly enough
- Mammals require specialised exchange surfaces and a mass transport system to deliver oxygen efficiently to all cells
Key terms in this question
Related
- All OCR A-Level Biology A (H420) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Exchange surfaces questions
- Explain why a large multicellular organism cannot rely solely on diffusion acros…
- Explain why a large multicellular organism, such as a mammal, requires a special…
- Explain why a large multicellular organism, such as a mammal, requires a special…
- Explain why the gills of a fish are an effective exchange surface for gas exchan…