A membrane that is permeable to certain types of MOLECULE but not others is called semipermeable. The critical determinant of passage is the size of the molecule, indexed by MOLECULAR WEIGHT. Semipermeable membranes may be natural or artificial: the...

A solid (usually thin) barrier that will allow the passage of smaller molecules from one side to the other but will prevent the passage of larger molecules. Semi-permeable membranes are usually used to separate dissolved solids from solution. See also:...

A semipermeable membrane, also termed a selectively-permeable membrane, a partially-permeable membrane or a differentially-permeable membrane, is a membrane that will allow certain molecules or ions to pass through it by diffusion and occasionally...