1.6 Diffusion

A free GCSE Biology lesson for AQA (8461) from OBA School: how particles spread out, where diffusion happens in living things, and the factors that affect its rate. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Define diffusion in terms of the net movement of particles down a concentration gradient
• Give examples of diffusion in animals and plants
• Explain how concentration gradient, temperature and surface area affect the rate of diffusion
• Interpret data on rates of diffusion and calculate a rate

Key words

• Diffusion: The spreading out of the particles of a gas or of any substance in solution. There is a net movement from an area of higher concentration to an area of lower concentration.
• Concentration: How much of a substance is in a given volume.
• Concentration gradient: The difference in concentration between two areas. A steeper (bigger) gradient means faster diffusion.
• Net movement: The overall movement once particles moving in both directions are taken into account.
• Partially permeable membrane: A membrane that lets some small molecules through but not others. The cell membrane is one.

What is diffusion?

Particles in a gas or a solution move about randomly all the time. When there are more particles in one place than another, more particles move away from the crowded area than move into it. Overall, they spread out. This net movement from higher to lower concentration is diffusion.

Key fact: Diffusion is passive: it does not need energy from respiration. The particles’ own random movement (kinetic energy) is enough.

Diffusion in living organisms

Small molecules such as oxygen, carbon dioxide, glucose, water and urea can diffuse through cell membranes. Large molecules such as starch and proteins cannot.

• Oxygen; Where it diffuses from the air in the lungs (alveoli) into the blood; from blood into respiring cells; Why it matters needed for aerobic respiration
• Carbon dioxide; Where it diffuses from respiring cells into the blood; from blood into the alveoli; into leaves through the stomata; Why it matters waste product of respiration; raw material for photosynthesis
• Urea; Where it diffuses from liver cells into the blood plasma, then carried to the kidneys; Why it matters waste product removed in urine
• Digested food (e.g. glucose); Where it diffuses from the small intestine into the blood; Why it matters used by cells for respiration

Factors that affect the rate of diffusion

• Concentration gradient; Effect bigger difference → faster; Reason many more particles move down the gradient than up it
• Temperature; Effect higher temperature → faster; Reason particles have more kinetic energy, so they move faster
• Surface area of membrane; Effect larger area → faster; Reason more particles can cross at the same time

A shorter distance (thinner membrane) also speeds up diffusion. Many exchange surfaces are folded to increase surface area. For example, the cells lining the small intestine have tiny folds called microvilli.

Exam tip: In 6-mark questions, name the factor, say how it changes the rate (“the steeper the gradient, the faster the rate”) and then explain why in terms of particles.

Investigating the rate of diffusion

A student put a coloured crystal in a dish of water and timed how long the colour took to spread 20 mm at different temperatures. Her results are plotted below.

Worked example: calculating a rate

At 40 °C the colour spread 20 mm in 150 s.
rate = distance ÷ time = 20 ÷ 150 = 0.133… mm/s

Answer: 0.13 mm/s (2 s.f.)

Common mistakes

• Saying particles move “along” a concentration gradient. Say they move down the gradient, from higher to lower concentration.
• Saying diffusion stops when the concentrations are equal. Particles keep moving; there is just no net movement.
• Saying diffusion needs energy (ATP or respiration). It is passive.
• Writing “the particles move faster” for surface area. A larger area means more particles can cross at once; speed of particles only changes with temperature.

Revision questions

Try these, then check your answers below each one.

Question 1. Which is the best definition of diffusion? (Choose from: Movement of water across a membrane, Net movement of particles from a higher to a lower concentration, Movement of particles against a concentration gradient using energy, Movement of particles from a lower to a higher concentration.)

Answer: Net movement of particles from a higher to a lower concentration

Working: Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration.

Question 2. True or false? Diffusion needs energy from respiration.

Answer: False

Working: False. Diffusion is passive; the particles move randomly using their own kinetic energy.

Question 3. Which change would increase the rate of diffusion? (Choose from: Decreasing the temperature, Increasing the temperature, Decreasing the surface area, Making the concentration gradient less steep.)

Answer: Increasing the temperature

Working: At higher temperature particles have more kinetic energy, so they diffuse faster.

Related lessons

• Previous lesson: 1.5 Specialisation in Plant Cells
• Next lesson: 1.7 Osmosis and Osmosis in Plant Cells
• In this chapter: 1.1 The World of the Microscope
• In this chapter: 1.2 Animal and Plant Cells
• In this chapter: 1.3 Eukaryotic and Prokaryotic Cells

Continue in the full course

This lesson is section 1.6 (AGCBH0106) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.