How are membranes classified according to their permeability?

Cell membranes often consist of a phospholipid bilayer with embedded, integral, and/or peripheral proteins responsible for communication and transportation of chemicals and ions. The permeability of a membrane is defined as the rate of passive diffusion of molecules through the membrane. Permeability depends mainly on the electric charge and polarity of the molecule and to a lesser extent the molar mass of the molecule. Membranes can be classified as impermeable, semipermeable, selectively permeable, and permeable membrane.

An impermeable membrane does not allow any substances to pass through. On the other hand, a selectively permeable membrane allows only specific solutes pass through while blocking others. For a permeable membrane, both solutes and solvent molecules are allowed to pass through. An excellent example of solute and solvent molecules is the passing of both ions and molecules. Finally, semipermeable membranes are those which only let solvents, such as water, pass through them.