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What is diffusion?

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient.

Diffusion explained

It is passive and driven by the random motion of particles. Rate rises with a steeper gradient, higher temperature, larger surface area and shorter diffusion distance.
Exchange surfaces such as alveoli and villi are adapted for exactly those factors: large area, thin walls and a good blood supply to maintain the gradient.

Key formula

rate ∝ (surface area × concentration difference) ÷ distance

Worked example

Oxygen diffuses from alveolar air into the blood across a wall just two cells thick.

Examiner tip

Fick's law names three factors. Quote all three when explaining an adaptation — each is usually a separate mark.

Diffusion: common questions

What is facilitated diffusion?
Passive movement of larger or charged particles through channel or carrier proteins, still down the concentration gradient and still without ATP.
Does diffusion stop at equilibrium?
Particles keep moving randomly, but there is no further net movement because the flows in both directions are equal.

Related terms

Osmosis

Osmosis is the net movement of water molecules from a dilute solution to a concentrated one through a partially permeable membrane.

Active transport

Active transport moves substances against a concentration gradient using carrier proteins and energy from ATP.

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