Diffusion occurs whenever random molecular motion causes molecules to move and mix together. This random motion is powered by the heat energy present in the surrounding environment. The rate of diffusion -- which causes molecules to naturally move from high concentration to low concentration in search of uniform distribution or "equilibrium" -- depends on several factors.
Molecules in Motion
Six physical and environmental conditions govern the rate of diffusion. Four of these are applicable to all types of diffusion, and two apply only to diffusion through a membrane. The mass of molecules is a major factor, because smaller molecules have higher random velocity for a given ambient temperature, and higher random velocities correspond to faster diffusion. Similarly, ambient temperature affects diffusion because higher temperatures lead to higher random velocities. Diffusing molecules flow from high concentration to low concentration, and the rate of diffusion increases when the difference in concentration is greater. The rate of diffusion decreases, however, when molecules must travel a longer distance in their search for equilibrium.
The two factors that are particular to diffusion through a membrane are surface area and permeability. A membrane with smaller surface area or lower permeability will impede molecular movement and thus lead to slower diffusion.