Cell membranes are selective barriers that separate individual cells and cellular compartments.
Membranes are assemblies of carbohydrates, proteins, and lipids held together by non-covalent forces. They regulate the transport of molecules, control information flow between cells, generate signals to alter cell behavior, contain molecules responsible for cell adhesion in theÂ
formation of tissues, and can separate charged molecules for cell signaling and energy generation.
Cell membranes are dynamic, constantly being formed and degraded. Membrane vesicles move between cell organelles and the cell surface. Inability to degrade membrane components can lead to lysosomal storage diseases.
Lipids of cell membranes include phospholipids composed of glycerol, fatty acids, phosphate, and a hydrophobic organic derivative such as choline or phosphoinositol. Cholesterol is a lipid component of cell membranes that regulates membrane fluidity and is a part of membrane signaling systems. The lipids of membranes create a hydrophobic barrier between aqueous compartments of a cell. The major structure of the lipid portion of the membrane is a lipid bilayer with hydrophobic cores made up predominately of fatty acid chains, and hydrophilic surfaces.
Membrane proteins determine functions of cell membranes, including serving as pumps, gates, receptors, cell adhesion molecules, energy transducers, and enzymes. Peripheral membrane proteins are associated with the surfaces of membranes while integral membrane proteins are embedded in the membrane and may pass through the lipid bilayer one or more times.
Carbohydrates covalently linked to proteins (glycoproteins) or lipids (glycolipids) are also a part of cell membranes, and function as adhesion and address loci for cells.
The Fluid Mosaic Model describes membranes as a fluid lipid bilayer with floating proteins and carbohydrates.
Cell junctions are a special set of proteins that anchor cells together (desmosomes), occlude water passing between cells (tight junctions), and allow cell to cell direct communication (gap junctions).
