For an organism to function, substances must move into and out of cells. Three processes contribute to this movement - diffusion, osmosis and active transport.
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 therefore move from this region to where they are in a lower concentration. This is called diffusion.
It is important to remember that the particles:
Some substances move into and out of living cells by diffusion.
In a leaf:
In the lungs:
In the liver: