membrane

membrane structure composed mostly of lipid and protein that forms the external boundary of cells and of major structures within cells. Membrane organization is based on a sheet two molecules thick—a double layer of lipids aligned with their long hydrocarbon tails tucked inside—studded with protein molecules, some of which extend completely through the lipid bilayer. The basic function of the membrane is to provide for the integrity of the cell—e.g., to separate the outside from the inside. While water and a few substances, such as carbon dioxide and oxygen, can diffuse across the membrane, most molecules necessary for cellular functions traverse the membrane by means of transport mechanisms. There are several such mechanisms and they rely upon interactions between a transportable molecule and specific protein molecules in the membrane. Among these is the Na ⁺ -K ⁺ pump, by which sodium ions within the cell are exchanged with potassium ions from without. Such transport functions permit selective entry of particular materials into the cell and into structures within the cell. Information can also be transmitted across the membrane. In this case, specific membrane proteins called receptors bind hormones or other such informational molecules and subsequently transmit a signal to the interior of the cell. Endocytosis also allows the bulk transport of materials across the membrane.

membrane In biology, boundary layer or layers inside or around a living cell or tissue. Cell membranes include the plasma membrane surrounding the cell, the network of membranes inside the cell (endoplasmic reticulum), and the double membrane surrounding the nucleus. The multicellular membranes of the body comprise mucous membranes of the respiratory, digestive and urinogenital passages, synovial membranes of the joints, and the membranes that coat the inner walls of the abdomen, thorax and the surfaces of organs. See also epithelium; synovial fluid

membrane
1. A thin sheet of tissue or other material that lines a body cavity, forms a partition, or connects various structures.

2. Any of the various flexible sheetlike structures, composed predominantly of lipids and proteins, that occur in living cells, such as the plasma membrane forming the cell boundary. See cell membrane.