Diffusion

Diffusion is the movement of a substance from an area of high concentration to an area of low concentration (down a concentration gradient). The concentration gradient is the difference in solute concentration between two points divided by the distance separating the points. The rate of diffusion increases with an increase in the concentration gradient, an increase in temperature, a decrease in molecule size, and a decrease in viscosity. The end result of diffusion is a uniform distribution of molecules. Diffusion requires no expenditure of energy. Small, uncharged molecules such as O₂, CO₂, H₂O, and lipid-soluble substances get transported across the membrane through the process of diffusion.