91�ȱ�

Diffusion is the random movement of a substance from a region of high concentration to a region of low concentration (ie: down the concentration gradient).

For example, oxygen moves from a high concentration in the lungs to a low concentration in the blood.

Substances (oxygen, carbon dioxide, water, dissolved nutrients and urea) can move into and out of a cell or organism via diffusion.

This happens across the cell membrane which is selectively permeable.

Rate of diffusion

The rate of diffusion is affected by the:

• Concentration gradient - a larger difference in the high and low concentrations increases the diffusion rate.
• Temperature - a higher temperature provides the particles with more kinetic energy increasing the diffusion rate.
• Surface area - a larger surface area through which diffusion can happen increases the diffusion rate.