Explain why the rate of diffusion of oxygen into a respiring cell increases when the cell uses oxygen more rapidly.

Pearson Edexcel International GCSE Biology (4BI1) — 2.4 Movement of substances into and out of cells · Explain · 4 marks · View as Markdown

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

Muscle cells respire rapidly during exercise, and oxygen must diffuse from the blood plasma into the muscle cells to meet the demand.

Model answer (4 marks)

Rapid respiration removes oxygen from the cell, lowering its internal concentration. This creates a steeper concentration gradient between the blood plasma and the cell. Diffusion is the net movement of particles from a higher to a lower concentration, so a steeper gradient means a faster rate of oxygen entering the cell.

Examiner tips

Use the word ‘gradient’ and explain it as a difference in concentration.
Show the link: lower inside concentration → steeper gradient → faster diffusion.
Mention that diffusion is always down the gradient.
Keep the answer concise and use the exact terms from the mark scheme.

Common mistakes

Failing to mention that the internal concentration is lowered by respiration.
Using vague terms like ‘more oxygen’ instead of ‘steeper gradient’.
Not explaining that diffusion is a net movement down the concentration gradient.

Mark scheme (4 marks)

Rapid respiration uses up / removes oxygen inside the cell, lowering the concentration of oxygen inside the cell
This increases / steepens the concentration gradient between the blood plasma (outside) and the cell (inside)
Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration (down the concentration gradient)
A steeper concentration gradient results in a faster / greater rate of diffusion into the cell

Key terms in this question

diffusion

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