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:
Diffusion is crucial in both single-celled and multi-celled organisms. In both single-celled organisms like prokaryotic bacteria and eukaryotic algae, oxygen and carbon dioxide molecules move by diffusion through cell membranes. Larger multi-cellular organisms such as vertebrates have specially adapted exchange surfaces, like lungs or gills. In these organisms, diffusion is still vital because it is this process that allows carbon dioxide and oxygen to move between the exchange surfaces.
Some substances move into and out of living cells by diffusion.