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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

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

In a leaf

In the lungs

Liver cells