see also : [[cells + movement across membranes]] [[osmosis]] [[active transport]]
Diffusion is the net movement of molecules from an area of high concentration to an area of low concentration down a concentration gradient. This is a passive process and does not require energy. Small molecules (e.g. O2 and CO2) move across membranes by diffusion. Factors affecting the rate of diffusion include: ● Temperature - the higher the temperature, the more kinetic energy possessed by molecules and the faster the rate of diffusion. ● Concentration gradient - the steeper the concentration gradient, the faster the rate of diffusion. Visking tubing (permeable, plastic tubing) can be used as a model of living material to investigate the rate of diffusion.