Explain why a single-celled organism does not need a specialised exchange surface or a circulatory system, but a multicellular organism does.

Edexcel GCSE Biology (1BI0) — 8.1 Gas exchange · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

A single‑cell organism has a very large surface area to volume ratio, so diffusion can rapidly supply all parts of the cell with oxygen and nutrients and remove waste.

A multicellular organism has a much smaller surface area to volume ratio; diffusion alone cannot reach all cells quickly enough, so a specialised exchange surface and a circulatory system are required to transport gases and nutrients.

Examiner tips

Use the exact terms ‘surface area to volume ratio’, ‘diffusion’, ‘specialised exchange surface’ and ‘circulatory system’.
Show the cause–effect link: large ratio → diffusion sufficient; small ratio → diffusion insufficient, hence need for transport system.
Keep the answer concise – one sentence per point – to fit the 4 marks.

Common mistakes

Confusing ‘surface area to volume ratio’ with ‘volume to surface area ratio’.
Mentioning ‘metabolism’ instead of ‘diffusion’ as the transport mechanism.
Writing a long paragraph instead of separate points, losing clarity.

Mark scheme (4 marks)

A single-celled organism has a large surface area to volume ratio
So diffusion alone is sufficient / fast enough to supply all parts of the cell with oxygen and nutrients and remove waste products
A multicellular organism has a small surface area to volume ratio
So diffusion alone is insufficient to supply all cells with required oxygen/nutrients and remove all waste products, requiring a specialised exchange surface and mass transport system

Key terms in this question

exchange surface · multicellular organism · single-celled organism