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

AQA GCSE Biology (8461) — 4.1.3 Transport in cells · Explain · 4 marks · View as Markdown

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

The alveoli have a large surface area and are surrounded by a dense network of capillaries. The blood arriving at the alveoli has a low concentration of oxygen.

Model answer (4 marks)

Oxygen moves into the blood by diffusion.
Oxygen diffuses from the alveoli, where its concentration is higher, into the capillary blood, where its concentration is lower.
The movement is driven by the concentration gradient between the alveoli and the blood.
This is a passive process and requires no energy from the body.

Examiner tips

  • Use the word "diffusion" and mention the concentration gradient. Show the direction of movement (alveoli → blood). State that it is passive and no energy is needed.
  • common_mistakes
  • :
  • Confusing diffusion with active transport. Forgetting to mention the concentration gradient or the direction of movement. Saying that oxygen requires energy to move into the blood.

Mark scheme (4 marks)

  1. Oxygen moves by diffusion
  2. Oxygen moves from a higher concentration in the alveoli to a lower concentration in the blood
  3. There is a concentration gradient between the alveoli and the blood
  4. No energy (from respiration) is required / it is a passive process

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