A
semipermeable membrane, also termed a
selectively permeable membrane (or a differentially or partially permeable membrane), is a type of
biological or
synthetic,
polymeric membrane that will allow certain
molecules or
ions to pass through it by
diffusion—or occasionally by more specialized processes of
facilitated diffusion, or
passive or
active transport. The rate of passage depends on the
pressure,
concentration, and
temperature of the molecules or
solutes on either side, as well as the permeability of the membrane to each solute. Depending on the membrane and the solute, permeability may depend on solute size,
solubility, properties, or chemistry. How the membrane is constructed to be selective in its permeability will determine the rate and the permeability. Many natural and synthetic materials thicker than a membrane are also semipermeable. One example of this is the thin film on the inside of an
egg.