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.
Diffusion 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
| location | particles move | from | to |
|---|---|---|---|
| gut | digested food products | gut cavity | blood in capillary of villus |
| lungs | oxygen | alveolar air space | blood circulating around the lungs |
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