The rate of diffusion is proportional to the surface area and the concentration gradient and is inversely proportional to the length of the diffusion path.Single celled organisms are small so they have a large surface area to volume ratio. Diffusion is fast enough and simple diffusion is sufficient to meet the cell's metabolic requirements. Oxygen can diffuse into the cells across the body surface and carbon dioxide can diffuse out. A cell wall is no additional barrier to diffusion.Diffusion is slower in water than in air.
The Rate of Diffusion
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Specification: AQA Biology (7401), AS-level, 3.3.1 Surface area to volume ratio. Topic: Exchange of Gases.
Key terms in this lesson
- diffusion
- The net passive movement of molecules or ions from a region of higher concentration to a region of lower concentration, down a concentration gradient.
- surface area to volume ratio
- The ratio of an organism's or structure's external surface area to its total volume, which decreases as size increases.
More in Exchange of Gases
- Tissue Fluid
- The Need for Specialised Exchange Surfaces
- Surface Area to Volume Ratio
- Features of A Specialised Exchange Surface
- Gas Exchange in Unicellular Organisms
- Gas Exchange in Insects
- Diffusion Gradient
- Mass Transport
All 28 lessons in Exchange of Gases · All AQA AS-level Biology topics