Diffusion
Get to grips with Diffusion for GCSE Biology with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Diffusion is the net movement of particles from a higher to a lower concentration — down a concentration gradient — and requires no energy.
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Topic overview
Diffusion is the spreading out of the particles of a gas or of a substance in solution, resulting in net movement from an area of higher concentration to an area of lower concentration.
It is a passive process, meaning it requires no energy from respiration. The particles move randomly, and the net movement simply results from there being more particles on one side to begin with.
Three factors affect the rate: the concentration gradient, the temperature, and the surface area of the membrane. Exchange surfaces in living organisms are adapted around exactly these factors — large surface area, thin membrane, and a good blood supply to maintain the gradient.
Revision notes
What diffusion is
Diffusion is the net movement of particles from a higher to a lower concentration, down the concentration gradient.
It is passive — no energy from respiration is needed. Oxygen and carbon dioxide move by diffusion in gas exchange, and urea diffuses from cells into the blood plasma.
Factors affecting the rate
A steeper concentration gradient gives a faster rate. A higher temperature gives a faster rate because particles have more kinetic energy and move faster.
A larger surface area of the membrane gives a faster rate because more particles can cross at once.
Adaptations of exchange surfaces
Effective exchange surfaces have a large surface area, a membrane that is thin so the diffusion path is short, and, in animals, an efficient blood supply.
In animals they are also ventilated. Each adaptation maps directly onto one of the rate factors, which is how these questions should be answered.
Key points
- Diffusion is net movement from high to low concentration.
- It is a passive process requiring no energy.
- A steeper concentration gradient increases the rate.
- A higher temperature increases the rate.
- A larger surface area increases the rate.
- Exchange surfaces are thin with a large surface area.
Worked examples
Example 1
Define diffusion. [2 marks]
Model answer
Diffusion is the spreading out of the particles of a gas, or of any substance in solution (1 mark), resulting in a net movement from an area of higher concentration to an area of lower concentration (1 mark).
Example 2
Explain why an increase in temperature increases the rate of diffusion. [2 marks]
Model answer
The particles gain more kinetic energy (1 mark), so they move faster and therefore spread out more quickly (1 mark).
Example 3
Explain how the alveoli are adapted for efficient gas exchange. [3 marks]
Model answer
They have a very large total surface area so many gas particles can diffuse at once (1 mark). Their walls are very thin, giving a short diffusion path (1 mark). They have a rich blood supply which maintains a steep concentration gradient (1 mark).
Common mistakes
Saying diffusion needs energy.
It is passive. Active transport is the process that requires energy.
Omitting the word net.
Particles move in both directions; it is the net movement that is down the gradient.
Describing the gradient backwards.
Movement is from high to low concentration, down the gradient.
Naming an adaptation without linking it to the rate.
Say a thin membrane gives a short diffusion path, not just that it is thin.
Exam tips
- Use the phrase net movement in your definition.
- Link each adaptation to the factor it affects.
- Say down the concentration gradient to be precise.
- Remember diffusion is passive; active transport is not.
Key terms
- Diffusion
- Net movement of particles from higher to lower concentration.
- Concentration gradient
- The difference in concentration between two areas.
- Passive
- Requiring no energy from respiration.
- Exchange surface
- A surface adapted for the efficient transfer of substances.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.