A
membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be
molecules,
ions, or other small particles.
Biological membranes include
cell membranes (outer coverings of cells or organelles that allow passage of certain constituents);
nuclear membranes, which cover a cell nucleus; and tissue membranes, such as
mucosae and
serosae.
Synthetic membranes are made by humans for use in
laboratories and industry (such as
chemical plants). The influent of an artificial membrane is known as the feed-stream, the liquid that passes through the membrane is known as the permeate, and the liquid containing the retained constituents is the retentate or concentrate.