Explain why a large multicellular organism cannot rely on diffusion alone to supply all of its cells with oxygen.

AQA A-Level Biology (7402) — 3.3.1 Surface Area to Volume Ratio · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

1. As an organism grows, its volume increases faster than its surface area.
2. Consequently, the surface area to volume ratio decreases.
3. A smaller ratio means less surface area is available per unit volume to absorb oxygen.
4. Cells located deep inside cannot receive sufficient oxygen by diffusion alone because the distance from the surface is too great.
5. Therefore, large organisms need specialised exchange surfaces or transport systems to supply all cells with oxygen.

Examiner tips

Use the exact phrase ‘surface area to volume ratio’ and explain the relationship between size and ratio. Show the logical chain: larger size → lower ratio → insufficient diffusion → need for transport systems.

Common mistakes

Confusing surface area with volume; writing ‘more surface area’ instead of ‘less surface area per unit volume’. Omitting the point that cells deep inside are too far from the surface for diffusion.

Mark scheme (5 marks)

As an organism gets larger, its volume increases more than its surface area
This means the surface area to volume ratio is smaller in a large organism
There is not enough surface area to absorb sufficient oxygen by diffusion for all the cells
Cells deep inside the organism are too far from the exchange surface for oxygen to reach them by diffusion alone
Large organisms require specialised exchange surfaces and/or transport systems to supply all cells with oxygen

Key terms in this question

diffusion · multicellular organism