Explain why a cell with a higher surface area to volume ratio absorbs dissolved substances by diffusion more quickly than a cell with a lower surface area to volume ratio.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Some unicellular organisms are very small, whilst some specialised animal cells are much larger. The rate at which substances move into a cell by diffusion depends on several factors, including the surface area to volume ratio of the cell.
Model answer (4 marks)
A higher surface area to volume ratio means more cell membrane is available relative to the volume of the cell, so more membrane is exposed to the surrounding solution.
This allows more substances to enter the cell across the membrane per unit time, because diffusion can occur across a greater area simultaneously.
The concentration gradient between the outside of the cell and the inside is maintained, so substances do not accumulate inside the cell as quickly relative to its volume.
Therefore the overall rate of diffusion into the cell is faster and substances reach all parts of the cell more quickly.
This allows more substances to enter the cell across the membrane per unit time, because diffusion can occur across a greater area simultaneously.
The concentration gradient between the outside of the cell and the inside is maintained, so substances do not accumulate inside the cell as quickly relative to its volume.
Therefore the overall rate of diffusion into the cell is faster and substances reach all parts of the cell more quickly.
Examiner tips
- Use the exact phrase "surface area to volume ratio" and link it to membrane area. Show that a greater area gives more simultaneous diffusion sites. Explain how the gradient is maintained. Finish with a clear causal statement: higher ratio → faster diffusion.
Common mistakes
- Confusing surface area to volume ratio with absolute surface area. Forgetting to mention the concentration gradient. Using vague terms like "more space" instead of "greater membrane area".
Mark scheme (4 marks)
- A higher surface area to volume ratio means more cell membrane is available relative to the volume of the cell / more membrane is exposed to the surrounding solution
- More substances can enter the cell across the membrane per unit time / diffusion occurs across a greater area simultaneously
- The concentration gradient between the outside of the cell and the inside is maintained / substances do not accumulate inside the cell as quickly relative to its volume
- Therefore the overall rate of diffusion into the cell is faster / substances reach all parts of the cell more quickly
Key terms in this question
surface area to volume ratio · diffusion
Related
- All Pearson Edexcel International GCSE Biology (4BI1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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