Explain why multicellular organisms need specialised exchange surfaces and transport systems, whereas single-celled organisms do not.

OCR GCSE Biology A: Gateway Science (J247) — B2.2 The challenges of size · Explain · 4 marks · View as Markdown

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

As organisms increase in size, the ratio of their surface area to their volume decreases. A single-celled organism such as an amoeba can exchange substances directly across its outer membrane, but a large multicellular organism such as a mammal cannot rely on simple diffusion across its outer surface to supply all of its cells.

Model answer (4 marks)

In a single‑cell organism the cell membrane is large compared with the cell’s volume, so all parts of the cell can be reached by simple diffusion across the membrane. In a multicellular organism the total surface area of the body is small compared with the total volume of all cells; diffusion across the outer surface is therefore too slow to supply every cell. To overcome this, multicellular organisms develop specialised exchange surfaces such as alveoli in lungs or root hairs in plants, which increase the total surface area for gas or nutrient exchange. In addition, transport systems such as the blood circulatory system or phloem move substances rapidly between these exchange surfaces and all cells, ensuring that every cell receives the required materials.

Examiner tips

  • Use the key terms: surface area, volume, diffusion, specialised exchange surfaces, transport systems
  • Show the cause–effect link: small SA:V ratio → diffusion insufficient → need for specialised surfaces and transport
  • Keep the answer concise – 4 points only, no extra explanation

Common mistakes

  • Confusing the need for specialised surfaces with the need for a circulatory system – both are required
  • Using vague phrases like ‘more efficient’ without specifying surface area or transport
  • Including irrelevant details such as photosynthesis or cellular respiration

Mark scheme (4 marks)

  1. In a single-celled organism, the surface area is large enough relative to its volume for diffusion to supply all parts of the cell
  2. Multicellular organisms have a small surface area relative to their volume, so diffusion across the outer surface is too slow / insufficient to meet the needs of all cells
  3. Specialised exchange surfaces (e.g. alveoli / root hair cells) increase the surface area to allow faster exchange of substances
  4. Transport systems (e.g. the blood / circulatory system / phloem) are needed to move substances quickly between exchange surfaces and all cells in the body

Key terms in this question

multicellular organism

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