Explain why a bacterium can obtain all the oxygen it needs by diffusion across its outer surface, but a mammal cannot rely on diffusion alone to supply oxygen to its cells.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A bacterium is a single-celled organism roughly 1–2 micrometres in diameter. A mammal is a large, multicellular organism with cells deep inside its body.
Model answer (5 marks)
A bacterium has a very large surface area to volume ratio, so its outer surface area is large compared with its volume and the number of cells it contains. This allows oxygen to diffuse across the surface and reach all of its cells in sufficient quantity and time.
In contrast, a mammal has a small surface area to volume ratio. The surface area is not large enough relative to its volume or the number of cells, so diffusion alone cannot supply all cells with enough oxygen. Many mammal cells are located far from the outer surface, too far for oxygen to reach by diffusion in sufficient quantities or within a reasonable time.
In contrast, a mammal has a small surface area to volume ratio. The surface area is not large enough relative to its volume or the number of cells, so diffusion alone cannot supply all cells with enough oxygen. Many mammal cells are located far from the outer surface, too far for oxygen to reach by diffusion in sufficient quantities or within a reasonable time.
Examiner tips
- Use the term ‘surface area to volume ratio’ and explain its effect on diffusion. Include the point that mammal cells are far from the surface. Keep the answer concise and to the points.
Common mistakes
- Confusing ‘surface area to volume ratio’ with ‘volume to surface area ratio’.
- Failing to mention that mammal cells are far from the surface.
Mark scheme (5 marks)
- A bacterium has a large surface area to volume ratio
- So the surface area is large enough relative to its volume / number of cells to supply sufficient oxygen by diffusion alone
- A mammal has a small surface area to volume ratio
- The surface area is not large enough relative to volume / the number of cells to supply all cells with sufficient oxygen by diffusion
- Many mammal cells are far from the outer surface / too far for oxygen to reach by diffusion in sufficient quantities / in time
Key terms in this question
Related
- All AQA A-Level Biology (7402) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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