Particles - molecules and ions - in a liquid and a gas move continuously. Because of this movement, particles will spread themselves evenly throughout a liquid or a gas.

If there is a situation where particles of a substance are in a higher concentration, they will move from this region to where they are in a lower concentration. This is called diffusion.

It is important to remember that the particles:

  • will move in both directions, but there will be a net movement from high to low concentration
  • will end up evenly spread throughout the liquid or gas, but will continue to move

Some examples of diffusion in biological systems

In a leaf

Some substances move into and out of living cells by diffusion.

Diagram showing the process of diffusion in a leaf

In the lungs

Carbon dioxide returns from the body in the blood and diffuses into the alveoli.

Oxygen diffuses into the blood from the alveoli and the blood carries it to all body cells.

Diagram showing how gases pass through the alveoli and oxygen is transported around the body

In the liver

Urea diffuses from the cells in the liver into the blood which carries it to the kidneys for excretion.

Some examples of diffusion in biological systems