A student investigates how the size of a cube affects the rate of diffusion into it. She uses agar cubes of different sizes soaked in acid. Explain why the smallest cube becomes fully coloured by the indicator solution more quickly than the largest cube.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Agar cubes of different sizes are placed in an indicator solution. The indicator diffuses into the agar from the outer surface. The smallest cube (1 cm sides) becomes fully coloured much faster than the largest cube (4 cm sides).
Model answer (5 marks)
The smallest cube has a larger surface area to volume ratio.
This means that, per unit volume, more of its surface is exposed to the indicator.
Consequently a greater amount of indicator can diffuse into the cube for a given time.
The largest cube has a smaller surface area to volume ratio, so less indicator enters per unit volume.
Moreover, the distance from the outer surface to the centre of the largest cube is greater.
Diffusion is a slow, passive process, so a longer diffusion distance takes more time, whereas the short distance in the smallest cube allows the whole cube to become coloured quickly.
This means that, per unit volume, more of its surface is exposed to the indicator.
Consequently a greater amount of indicator can diffuse into the cube for a given time.
The largest cube has a smaller surface area to volume ratio, so less indicator enters per unit volume.
Moreover, the distance from the outer surface to the centre of the largest cube is greater.
Diffusion is a slow, passive process, so a longer diffusion distance takes more time, whereas the short distance in the smallest cube allows the whole cube to become coloured quickly.
Examiner tips
- State the surface area to volume ratio first, then link it to diffusion rate.
- Explain the role of diffusion distance and note that diffusion is passive and slow.
- Use the word "indicator" and "coloured" to match the stimulus.
- Keep the answer to 5 clear points, each worth one mark.
Common mistakes
- Confusing surface area with volume alone.
- Failing to mention diffusion distance or that diffusion is passive.
- Using vague terms like "more surface" without quantifying the ratio.
Mark scheme (5 marks)
- The smallest cube has a larger surface area to volume ratio
- A larger surface area to volume ratio means more diffusion can occur relative to the volume of the cube
- In the largest cube the diffusion distance to the centre is greater
- Diffusion is a slow/passive process so a longer distance takes more time
- In the smallest cube the diffusion distance to the centre is short so the entire cube becomes coloured quickly
Key terms in this question
Related
- All AQA A-Level Biology (7402) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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