Diffusion (Biology)
Diffusion is the net movement of particles from a region of their higher concentration to a region of their lower concentration — down a concentration gradient — as a result of their random movement 1.
The energy for this movement comes from the kinetic energy of that random movement of the molecules and ions, so the cell supplies none of it and diffusion costs the organism nothing 1. Once the gradient is gone there is no net movement, although the particles keep moving randomly — the state OpenStax calls dynamic equilibrium 2.
Into and Out of Cells
Some substances move into and out of cells by diffusion through the cell membrane 1. Oxygen diffuses into cells and carbon dioxide diffuses out of them, so diffusion of gases is the mechanism behind gas exchange, while dissolved solutes are taken up in the same way, as in absorption from the small intestine 1.
Exchange surfaces are built around this. The alveoli of the lungs give a large surface area, a thin surface, a good blood supply and good ventilation for gas exchange 1, and the villi and microvilli of the small intestine increase the internal surface area available for absorption 1.
The Four Factors
At IGCSE the factors that influence the rate of diffusion are limited to four 1:
| Factor | Effect on diffusion |
|---|---|
| Surface area | A larger surface area makes diffusion faster |
| Temperature | A higher temperature makes diffusion faster |
| Concentration gradient | A steeper gradient makes diffusion faster |
| Distance | A shorter distance makes diffusion faster |
Where Diffusion Stops Being Enough
Diffusion can only move a substance down its gradient. When a cell needs something that is already more concentrated inside it — mineral ions in a root hair cell, for instance — the cell must spend energy on active transport, released by respiration 1. Water crosses membranes in its own way, by osmosis.
The underlying physics — Fick’s laws and the diffusion coefficient — belongs to the Chemistry area and is treated separately in Diffusion (Chemistry).
Related
- Diffusion (Chemistry) - The physical laws of diffusive transport.
- Osmosis - Diffusion of water only, across a partially permeable membrane.
- Active Transport - Energy-consuming movement against the gradient.
- Partially Permeable Membrane - Barrier whose pores let small particles through but not large ones.
- Cell Membrane - Partially permeable barrier that substances diffuse across.
- Root Hair Cell - Cell taking in soil water by osmosis and mineral ions by active transport.
- Biology - Cambridge IGCSE life-science area.
Footnotes
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Cambridge IGCSE Biology 0610 syllabus for examination in 2026, 2027 and 2028 - Diffusion as net movement down a concentration gradient (3.1.1), kinetic energy of random movement as the energy source (3.1.2), substances entering and leaving cells by diffusion through the cell membrane (3.1.3), importance of diffusion of gases and solutes in organisms (3.1.4), factors influencing diffusion: surface area, temperature, concentration gradient and distance (3.1.5); features of gas exchange surfaces (11.3.1); villi and microvilli increasing the internal surface area of the small intestine (7.4.3); active transport and ion uptake by root hairs (3.3.1-3.3.2). ↩ ↩2 ↩3 ↩4 ↩5 ↩6 ↩7 ↩8
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OpenStax Biology 2e - Passive Transport - Factors affecting the diffusion rate, including concentration gradient, temperature, mass, surface area and distance, and the idea of dynamic equilibrium. ↩