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Particles 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. Diffusion is a spreading out and mixing process.

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

Diffusion is an essential process in cells. They need glucose and oxygen for respiration, which move into them by diffusion. Respiration produces carbon dioxide and water which leave cells by diffusion. (Water actually leaves cells by osmosis, which is the diffusion of water across a membrane.)

Some examples of diffusion in biological systems

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

In a leaf

In the lungs

Liver cells