Diffusion — AQA GCSE Biology
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Diffusion explained
Diffusion is the passive spreading out of particles.
Read the full explanation
It applies to particles of a substance in solution and to particles of a gas. The particles move randomly in all directions, but if one region has a higher concentration than another, there are more particles moving away from the high-concentration region than into it. This produces a net movement from higher concentration to lower concentration. Eventually the concentrations become equal and there is no net movement, although particles continue to move randomly. A familiar example is a drop of ink spreading through water until the colour is even. The statement is assessed by asking you to define diffusion precisely, including the type of particles and the direction of net movement.
Your focus
- Define diffusion in terms of particles in solution or gas.
- Explain how random particle movement produces a net movement from higher to lower concentration.
- Recognise that diffusion is passive and continues until concentrations are uniform.
Diffusion exam tips
Marking Points
- Diffusion involves the spreading out of particles of a substance in solution or particles of a gas.
- Particles move randomly, which causes them to spread from where they are more concentrated.
- There is a net movement from an area of higher concentration to an area of lower concentration.
- Net movement means the overall movement, even though individual particles move in all directions.
- Diffusion continues until the concentration is uniform, after which there is no net movement.
- Diffusion is a passive process and does not require energy from respiration.
Examiner Tips
- 💡Use the words net movement and concentration gradient to make the definition precise.
- 💡State that particles are in solution or in a gas, because this matches the specification wording.
- 💡If asked to explain, link random particle motion to the overall net direction.
Common Mistakes
- Defining diffusion as movement only from high to low concentration without mentioning random particle movement; the correction is to include both the random motion and the net direction.
- Saying particles stop moving when concentrations are equal; the correction is that particles continue to move randomly but there is no net movement.
- Applying diffusion only to gases; the correction is that particles of substances in solution also diffuse.