Diffusion

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FreeScienceLessons · AQA BiologyDiffusion — definition, factors affecting rate

Key facts

Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration
It is a passive process — no energy (ATP) is required
Movement is down the concentration gradient
Applies to: gases (O₂, CO₂) and dissolved substances (glucose, urea)
Mark scheme: do NOT say "movement from high to low" without saying "net" — particles move both ways, but the net movement is down the gradient

Steeper concentration gradient — bigger difference between high and low
Higher temperature — particles have more kinetic energy, move faster
Larger surface area — more area for particles to cross
Shorter diffusion distance — thin membranes/walls mean shorter path
These four factors explain adaptations of exchange surfaces (alveoli, villi, root hair cells)

Exam questions — 5 questions · 9 marks

1 markDefinitionAQA 8461 style

Which of the following is the correct definition of diffusion?

A The movement of particles using energy, from low to high concentration

B The net movement of particles from an area of higher concentration to an area of lower concentration

C The movement of water molecules across a partially permeable membrane

D The movement of particles using ATP from high to low concentration

2 marksFactors affecting rateAQA Nov 2020 P1 style

Oxygen diffuses from the alveoli into the blood. Give two features of alveoli that increase the rate of diffusion. (2 marks)

+20 XP

1 markDirection of diffusionAQA Jun 2024 P1 style

Carbon dioxide concentration is higher inside a respiring cell than outside. In which direction will CO₂ diffuse?

A Into the cell, from low to high concentration

B Out of the cell, from high to low concentration

C In both directions equally — no net movement

D Into the cell, using energy

3 marksExchange surfacesAQA Nov 2020 P1 Q2 style

Explain how the alveoli are adapted to maximise the rate of gas exchange by diffusion. (3 marks)

+30 XP

2 marksEffect of temperatureAQA Jun 2024 P1 style

Explain why increasing temperature increases the rate of diffusion. (2 marks)

+20 XP