GCSE Biology · Edexcel 1BI0 · Topic 8A
Gas exchange
Key points
- Organisms exchange substances such as oxygen, carbon dioxide, food and urea with their surroundings.
- Surface area : volume ratio falls as an organism gets bigger, so large organisms need specialised exchange surfaces and transport systems.
- Alveoli are adapted for gas exchange: a huge surface area, walls one cell thick (short diffusion distance), a rich blood supply and ventilation that maintain a steep concentration gradient.
- Oxygen diffuses from the air in the alveoli into the blood; carbon dioxide diffuses the other way.
- Fick's law: rate of diffusion .
- Calculate SA : V for a cube, e.g. a 2 cm cube has 24 cm² ÷ 8 cm³ = 3 : 1.
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Every cell needs oxygen and must get rid of carbon dioxide. This sub-topic explains why large organisms cannot rely on diffusion through their outer surface and how the alveoli in the lungs are adapted to exchange gases quickly.
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