Diffusion — Cambridge IGCSE Biology
Diffusion is the net movement of particles down a concentration gradient; learn why it matters and what changes its rate.
Revision notes
2 short notes, in the order of the specification. Each one in short:
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. The energy comes from the kinetic energy of the moving molecules and ions, so the cell supplies none. Oxygen, carbon dioxide and water diffuse through cell membranes.
The rate of diffusion depends on four factors: surface area, temperature, concentration gradient and distance. Diffusion is faster with a larger surface area, a higher temperature, a steeper concentration gradient and a shorter distance. In agar cubes containing indicator placed in dilute acid, the smallest cube, with the largest surface area compared with its volume, loses its colour first.
Exam questions
6 exam-style questions (16 marks), each with its mark scheme.
Answer the questionsFlashcards
7 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of movement into and out of cells on one page, so you can see where this subtopic fits.
Open the mind mapPractical sheet
Method, variables, risks and a results table, with questions to answer.
Print it
Free PDFs to print or save.
Quick check questions
Describe diffusion.
Show answer
The net movement of particles from a region of their higher concentration to a region of their lower concentration (i.e. down a concentration gradient), as a result of their random movement.
Where does the energy for diffusion come from?
Show answer
The kinetic energy of the random movement of molecules and ions.
Name two substances that move into or out of cells by diffusion.
Show answer
Any two from: oxygen, carbon dioxide, urea, digested food molecules.
Name the four factors you investigate that affect the rate of diffusion.
Show answer
Surface area, temperature, concentration gradient and distance.
Frequently asked questions
Does diffusion need energy from the cell?
No. Diffusion is passive, so the cell does not supply any energy of its own. The energy comes from the kinetic energy of the random movement of molecules and ions. Particles move in both directions, but more move from the region of higher concentration, giving a net movement down the concentration gradient.
Why is the surface area to volume ratio important for cells?
A larger surface area to volume ratio means faster diffusion into the whole cell. In the agar cube experiment, the smallest cube turns colourless first because it has more surface for its volume and a shorter distance to the centre. A 1 cm cube has a ratio of 6 : 1, but a 2 cm cube only 3 : 1.
How does temperature affect the rate of diffusion?
A higher temperature increases the rate of diffusion because the particles have more kinetic energy and move faster. You can test this by placing agar cubes of the same size in dilute hydrochloric acid at different temperatures, such as 20, 30, 40 and 50 °C, and timing how long each takes to turn colourless.
Why is diffusion important in living organisms?
Diffusion moves substances into and out of cells without the cell using energy. Oxygen diffuses into cells for aerobic respiration and carbon dioxide diffuses out. In leaves, carbon dioxide diffuses in for photosynthesis. Digested food molecules diffuse from the gut into the blood, and urea diffuses from cells into the blood plasma.
Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026