Main Page | See live article | Alphabetical index A semi-permeable membrane is a membrane which will allow the passage of certain molecules through it by the process of diffusion. The rate of passage depends on the pressure, concentration and temperature of the solvent on either side, as well as the permeability of the membrane to each type of molecule or solute. Depending on the membrane and the solute, permeability may depend on solute size, solubility properties, or chemistry. An example of a semi-permeable membrane is a lipid bilayer, on which is based the plasma membrane that surrounds all biological cells. Many natural and synthetic materials thicker than a membrane are also semi-permeable. This article is from Wikipedia. All text is available under the terms of the GNU Free Documentation License.