Within cells the two main forms of cellular exchange are diffusion and osmosis. These two terms can sometimes be mixed up as they both mean "the movement of molecules of a substance from an area of higher concentration to an area of lower concentration." The difference between the two processes is that where DIFFUSION is just the movement of molecules of a substance from an area of higher concentration to an area of lower concentration; OSMOSIS is specifically the movement of molecules of WATER through a SELECTIVELY PERMEABLE MEMBRANE from an area of higher water concentration to an area of lower water concentration.