Explain why oxygen moves from the alveoli into the blood in the lungs, and why carbon dioxide moves in the opposite direction, from the blood into the alveoli.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Gas exchange in the lungs relies on the movement of gases across cell membranes. Oxygen and carbon dioxide both move by the same transport process.
Model answer (5 marks)
Both gases move by diffusion across the partially permeable alveolar‑capillary membrane. Oxygen diffuses from the alveoli into the blood because the oxygen concentration in the alveolar air is higher than that in the blood. Carbon dioxide diffuses in the opposite direction because its concentration in the blood is higher than that in the alveolar air. Neither process requires energy – diffusion is a passive process.
Examiner tips
- State that diffusion is the transport mechanism; mention the membrane; give the concentration gradients for O₂ and CO₂; note that diffusion is passive.
- Use the exact terms ‘alveolar‑capillary membrane’, ‘higher concentration’, and ‘passive’ to match the mark scheme.
Common mistakes
- Confusing the direction of diffusion for each gas; writing that O₂ moves into the alveoli or CO₂ into the blood; failing to mention that diffusion is passive or that it occurs across the membrane.
Mark scheme (5 marks)
- Both gases move by diffusion
- Oxygen moves from alveoli to blood because oxygen concentration is higher in the alveoli than in the blood
- Carbon dioxide moves from blood to alveoli because carbon dioxide concentration is higher in the blood than in the alveoli
- Gases move across the partially permeable cell membrane of the alveolar / capillary wall
- No energy is required / diffusion is a passive process (linked to context)
Related
- All Edexcel A-Level Biology A: Salters-Nuffield (9BN0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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