How a membrane works
Semi-permeable membranes - General

An artificial membrane with selective permeability is a material barrier which, under the action of an acting force, allows substances to be selectively stopped or passed between the two volumes it separates . The permeability of the membrane, characterized by the transmembrane flux, is directly related to the intensity of this force.

Depending on whether the force consciously applied is due to a pressure gradient, an electrical potential gradient or a chemical potential gradient, often...