# 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.

> Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 1.6 Membranes and transport · Explain · 5 marks

> 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.

## Mark scheme (5 marks)

1. Both gases move by diffusion
2. Oxygen moves from alveoli to blood because oxygen concentration is higher in the alveoli than in the blood
3. Carbon dioxide moves from blood to alveoli because carbon dioxide concentration is higher in the blood than in the alveoli
4. Gases move across the partially permeable cell membrane of the alveolar / capillary wall
5. No energy is required / diffusion is a passive process (linked to context)

## Related

- [Revision notes for Edexcel A-Level Biology A: Salters-Nuffield (9BN0)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-oxygen-moves-from-the-a957987b) · Published by Druglandscape Ltd.