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A semipermeable membrane is a membrane that allows certain types of molecules to pass through but blocks others. Body cells are surrounded by this type of membrane, which helps to control what substances can and cannot pass into the cells. By serving as a barrier between the interior and the exterior of the cell, it protects the cell from foreign bodies that could be harmful. Outside of the body, these membranes, usually artificially created, are used for specific functions such as water desalination and purification.
Other terms used to refer to a semipermeable membrane are "selectively permeable membrane," "partially permeable membrane," or "differentially permeable membrane." All of these terms simply refer to the fact that the barrier controls what molecules can and cannot pass through it, based on characteristics such as the molecules' size, chemistry, solubility, or other specific properties. Molecules pass through the membrane via osmosis, which is a term used to describe the passage of any substance through diffusion, or the tendency of molecules to move from an area where they exist in higher concentrations to an area of lower concentration.
Semipermeable membranes in cells also allow some molecules to pass through via facilitated diffusion. In this case, the molecules cannot pass through the membrane under normal conditions but must bond to another molecule first. This combination of molecules allows the original molecule to pass through the cell membrane and allows the cell to regulate the amount and type of individual molecules that enter it. In most cases, facilitated diffusion occurs through the action of a carrier protein that attaches to another molecule, such as a charged ion or a polar molecule, allowing it to pass through the cell wall.
One practical use of such membranes is reverse osmosis, often used in water purification. In reverse osmosis, an artificial membrane is used to remove undesirable substances from water using the natural actions of a semipermeable membrane and osmotic pressure. It often makes use of two membranes that are permeable to water but not permeable to other molecules, such as salt or other minerals. This process commonly is used to desalinate water or to purify well water, which can contain a number of chemicals or minerals leached into the water from the surrounding soil. It also is used to purify blood during dialysis.