Explain how oxygen moves from the air sacs (alveoli) in the lungs into the blood during breathing.

WJEC GCSE Biology (Wales) — 1.1.2 Movement of substances · Explain · 4 marks · View as Markdown

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

Oxygen is needed by body cells for respiration. It must cross from the air sacs in the lungs into the bloodstream before it can reach those cells.

Model answer (4 marks)

Oxygen moves from the air sacs to the blood by diffusion.
1. It travels from a region of high concentration (in the alveoli) to a region of low concentration (in the blood).
2. A concentration gradient therefore exists between the alveolar air and the capillary blood.
3. The movement is passive – no energy is required.
4. The gradient drives oxygen into the bloodstream until equilibrium is reached.

Examiner tips

  • Use the word ‘diffusion’ early. Show the direction of movement (high to low). Mention the concentration gradient. State it is a passive process – no energy needed.

Common mistakes

  • Confusing diffusion with active transport. Forgetting to mention the concentration gradient. Saying ‘oxygen moves into the blood’ without explaining the direction or the gradient.

Mark scheme (4 marks)

  1. The process by which oxygen moves is diffusion
  2. Oxygen moves from a high concentration (in the air sacs) to a low concentration (in the blood)
  3. There is a concentration gradient between the air sacs and the blood
  4. No energy is required / it is a passive process

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