Diffusion

This lesson covers diffusion — one of the key processes by which substances move in and out of cells — as required by the AQA GCSE Combined Science Trilogy specification (8464). You need to understand the definition, factors affecting the rate of diffusion and examples of diffusion in living organisms.

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What Is Diffusion?

Diffusion is the movement of particles from an area of higher concentration to an area of lower concentration, down a concentration gradient.

Key points:

• Diffusion is a passive process — it does not require energy from respiration.
• It happens because particles are in constant random motion (they move in all directions).
• Net movement occurs from high to low concentration until the particles are evenly distributed (equilibrium).
• Diffusion occurs in both liquids and gases (not in solids, as particles are fixed in position).

Exam Tip: Always refer to the "net movement" of particles. Individual particles move randomly in all directions, but the overall (net) movement is from high to low concentration. The word "net" is important for full marks.

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The Concentration Gradient