Diffusion

This lesson covers diffusion — one of the three transport mechanisms you need to know for the Edexcel GCSE Biology specification (1BI0), Topic 1: Key Concepts in Biology. You must be able to define diffusion, explain the factors that affect its rate, and give examples of diffusion in living organisms.

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What Is Diffusion?

Diffusion is the net movement of particles (molecules or ions) from a region of higher concentration to a region of lower concentration, down a concentration gradient.

Key characteristics:

• Diffusion is a passive process — it does not require energy from the cell (no ATP needed).
• It occurs because particles are in constant random motion (they move in all directions).
• Although individual particles move randomly, the net (overall) movement is from high to low concentration.
• Diffusion continues until the particles are evenly spread (equilibrium is reached), although particles continue to move in both directions at equal rates.

Exam Tip: The definition of diffusion is worth memorising word for word: "The net movement of particles from an area of higher concentration to an area of lower concentration, down a concentration gradient." This comes up regularly on Edexcel papers.

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