Explain why multicellular animals need a circulatory system, whereas single-celled organisms do not.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Single‑cell organisms have a high surface area to volume ratio, so diffusion can reach every part of the cell quickly enough to supply oxygen, nutrients and remove waste.
Multicellular organisms have a much lower surface area to volume ratio; diffusion alone would be too slow and insufficient to reach all cells.
Therefore a mass transport system – a circulatory system – is required to carry oxygen, nutrients and glucose rapidly to all tissues.
The circulatory system also removes waste products such as CO₂ and urea from cells that are far from the exchange surfaces.
Multicellular organisms have a much lower surface area to volume ratio; diffusion alone would be too slow and insufficient to reach all cells.
Therefore a mass transport system – a circulatory system – is required to carry oxygen, nutrients and glucose rapidly to all tissues.
The circulatory system also removes waste products such as CO₂ and urea from cells that are far from the exchange surfaces.
Examiner tips
- Use the exact phrases ‘surface area to volume ratio’ and ‘diffusion alone is insufficient’. Show the logical link: high ratio → diffusion works; low ratio → need circulation. Mention both supply and removal of substances. Keep answer concise to fit 4 marks.
- common_mistakes
- :
- Confusing ‘surface area to volume ratio’ with ‘volume to surface area ratio’. Forgetting to mention waste removal. Writing a long paragraph instead of clear, separate points.
Mark scheme (4 marks)
- Single-celled organisms have a large surface area to volume ratio, so diffusion alone is sufficient to supply all parts of the cell with oxygen and nutrients / remove waste products.
- Multicellular organisms have a small surface area to volume ratio, so diffusion alone is insufficient / too slow to supply all cells with oxygen and nutrients and remove waste products.
- A mass transport system / circulatory system is needed to carry substances (oxygen, nutrients, glucose) quickly to all cells throughout the body.
- The circulatory system also removes waste products (e.g. carbon dioxide / urea) from cells that are far from exchange surfaces.
Key terms in this question
circulatory system · single-celled organism
Related
- All Edexcel GCSE Biology (1BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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