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.

hyaline membrane disease: see infant respiratory distress syndrome.

In biology, the thin layer that forms the outer boundary of a living cell or of an internal cell compartment. The outer boundary is the plasma membrane, and the compartments enclosed by internal membranes are called organelles. Biological membranes have a dual function: separation of vital but incompatible metabolic processes conducted in the organelles; and passage of nutrients, wastes, and metabolic products between organelles and between the cell and the outside environment. Membranes consist largely of a double layer of lipids in which are embedded large proteins, many of which transport ions and water-soluble molecules across the membrane. Seealso cytoplasm, eukaryote.

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or hyaline membrane disease

Common complication in newborns, especially after premature birth. Symptoms include very laboured breathing, bluish skin tinge, and low blood oxygen levels. Insufficient surfactant in the pulmonary alveoli raises surface tension, hampering lung expansion. The alveoli collapse (see atelectasis), and a “glassy” (hyaline) membrane develops in the alveolar ducts. Once the leading cause of death in premature infants, the syndrome is now usually treated for a few days with a mechanical ventilator (see respiratory therapy), with no aftereffects. An adult respiratory distress syndrome (ARDS) can follow lung injury.

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