Bitesize has changed! We're updating subjects as fast as we can. Visit our new site to find Bitesize guides and clips - and tell us what you think!




Dissolved substances have to pass through the cell membrane to get into or out of a cell. Diffusion is one of the processes that allows this to happen.

Diffusiondiffusion: The movement of particles (molecules or ions) from an area of higher concentration to an area of lower concentration occurs when particles spread. They move from a region where they are in high concentration to a region where they are in low concentration. Diffusion happens when the particles are free to move. This is true in gases and for particles dissolved in solutions. Particles diffuse down a concentration gradient, from an area of high concentration to an area of low concentration. This is how the smell of cooking travels around the house from the kitchen, for example.

Examples of diffusion

Two examples of diffusion down concentration gradients

locationparticles movefromto
gutdigested food productsgut cavityblood in capillary of villus
lungsoxygenalveolar air spaceblood circulating around the lungs

Remember, particles continue to move from a high to a low concentration while there is a concentration gradient [concentration gradient: A difference in concentration between two areas next to each other. Particles will move down the concentration gradient from an area of high concentration to an area of low concentration. ].

In the lungs, the blood will continue to take in oxygen from the alveolar air spaces provided the concentration of oxygen there is greater than in the blood. Oxygen diffuses across the alveolar walls into the blood, and the circulation takes the oxygen-rich blood away.

Back to Cells index

BBC © 2014 The BBC is not responsible for the content of external sites. Read more.

This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.