the capacity of a membrane to allow some particles to pass through but not others. Such ‘differentially permeable’ membranes (e.g. CELL MEMBRANE, cellular organelle membrane, TONOPLAST) allow water molecules to pass readily through them, whereas solutes dissolved in water can pass less rapidly or not at all. The ability of molecules to travel across the membrane and the velocity with which they do so is dependent on the fat solubility, size, and charge of the molecules. An extreme example of differential permeability is a semipermeable membrane which is almost completely impermeable to solute molecules, but is permeable to the solvent.Such membranes, however, are rare.see ACTIVE TRANSPORT.