Temperature and particle size affect the amount of kinetic energy available for diffusion. Solution concentration, diffusion distance, and a membrane's surface area and permeability affect net diffusion without impacting the kinetic energy.
Diffusion occurs when molecules move randomly using thermal or kinetic energy. Molecules in a solution tend to achieve a state of equilibrium, becoming evenly distributed. In net diffusion, molecules move in a direction from higher to lower concentrations. If a membrane separates two areas of differing concentration of the substance, net diffusion occurs through the membrane.
Net diffusion rates reflect the speed to reach equilibrium, not the speed of individual molecules. Temperature changes directly correlate to diffusion rates. Higher temperatures increase the kinetic energy of a substance, speeding diffusion rates. Particle size inversely affects diffusion rates. Larger particles require more kinetic energy to move, causing lower rates of diffusion than smaller particles at the same temperature.
Solution concentration drives net diffusion. The higher the gradient, the more molecules must move to eliminate differences, increasing net diffusion. More dispersed particles must migrate longer distances before equilibrium is reached, making net diffusion slower.
Membranes have variable levels of permeability, slowing or speeding net diffusion for particular substances. Organisms use this characteristic to control the movement of chemicals through cellular spaces.