Particles - molecules or ions - in a liquid and a gas are constantly moving randomly. Because of this movement, particles will spread themselves evenly throughout.
If particles of a substance are in a higher concentration compared to another area, their random movement leads to a net movement of particles from this region to the region of lower concentration. This is called diffusion.
It is important to remember that the particles:
Cells carry out chemical reactions, such as respiration and synthesis of biomass. For example, in aerobic respiration cells need a supply of glucose and oxygen. The process also creates carbon dioxide, a toxic substance that needs to be removed from cells.
Diffusion is one of the processes that is used to get substances into and out of cells. Substances also need to enter or leave whole organisms and this often requires diffusion too.
Some substances move into and out of living cells by diffusion.
Carbon dioxide is needed in the leaf for photosynthesis, and so diffuses into the leaf from a region of higher concentration in the air.
Oxygen from the air passes into the bloodstream in the lungs through structures called alveoli. It diffuses to a region of lower concentration in the bloodstream.
The liver breaks down excess amino acids, caused by too much protein in the diet. This produces urea which is a toxic substance. Urea diffuses from a high concentration in liver cells to a lower concentration in the bloodstream.