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