Membrane
Membrane (Latin membrana,”parchment”), in biology, any thin layer of connective tissue coating individual cells and organs of the body, or lining the joints and the ducts and tracts that open to the exterior of the body. The membrane surrounding single-celled animals and plants and individual cells in multicellular organisms is important in the nutritive, respiratory, and excretory processes of these cells. Such cell membranes are semipermeable; that is, they allow the passage of small molecules, such as those of sugars and salts, but not large molecules, such as those of proteins. Structures inside cells, such as the nucleus, may also have membranes. Each organ in the animal body is surrounded by a membrane, extensions of which often anchor the organ to the body wall. Three membranes, known as meninges, surround the brain and spinal cord; the outermost is known as the dura mater, the middle layer as the arachnoid, and the innermost as the pia mater. Each lung is coated with a membrane