A Partially Permeable Membrane is a thin external cellular layer that selectively tolerates the entry of specific types of molecules and atoms by diffusion or by facilitated diffusion; but prevents the access of others. As a barrier of the internal cellular structures from the external environment, the cell is kept from damaging extraneous substances.

The rate of differential passage will rely on the membrane permeability, selective construction of the membrane, pressure, the amount and the relative degree of heat of the molecules or solutes on both peripheries. On the other hand, permeability is influenced by the molecular size, capacity to dissolve and chemical properties.

Reverse osmosis in purifying liquids is one of the realistic functions of partially permeable membrane. Well water containing many different chemicals is desalinated and purified, as well as blood in hemodialysis. In this diffusion, a man-made membrane with similar function with differentially permeable membrane and osmotic pressure is employed to take out unwanted materials from water. Two membranes are involved which are both permeable to H20 but does not allow substances like salt and minerals to get through.