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The membrane that surrounds the cytoplasm of a cell; it is also called the plasma membrane or, in a more general sense, a unit membrane. This is a very thin, semifluid, sheetlike structure made of four continuous monolayers of molecules. The plasma membrane and the membranes making up all the intracellular membranous organelles display a common molecular architectural pattern of organization, the unit membrane pattern, even though the particular molecular species making up the membranes differ considerably. All unit membranes consist of a bilayer of lipid molecules, the polar surfaces of which are directed outward and covered by at least one monolayer of nonlipid molecules on each side, most of which are protein, packed on the lipid bilayer surfaces and held there by various intermolecular forces. Some of these proteins, called intrinsic proteins, traverse the bilayer and are represented on both sides. The segments of the polypeptide chains of these transverse proteins within the core of the lipid bilayer may form channels that provide low-resistance pathways for ions and small molecules to get across the membrane in a controlled fashion. Sugar moieties are found in both the proteins and lipids of the outer half of the unit membrane, but not on the inside next to the cytoplasm. The molecular composition of each lipid monolayer making up the lipid bilayer is different. The unit membrane is thus chemically asymmetric. See Cell organization
The unit membrane of a cell is a continuous structure having one surface bordered by cytoplasm and the other by the outside world. It appears in thin sections with the electron microscope as a triple-layered structure about 7.5–10 nanometers thick consisting of two parallel dense strata each about 2.5 nm thick separated by a light interzone of about the same thickness. The plasma membrane may become tucked into the cytoplasm and pinch off to make an isolated vesicle containing extracellular material by a process called endocytosis. During endocytosis the membrane maintains its orientation, with its cytoplasmic surface remaining next to cytoplasm. In this sense the contents of intracellular organelles, such as the endoplasmic reticulum, Golgi apparatus sacs, nuclear membrane, lysosomes, peroxisomes, and secretion granules, are material of the outside world, since at some time the space occupied by this material may become continuous directly or indirectly with the outside world. Hence the surface of the membrane bordering such material and lying between it and cytoplasm is topographically an external membrane surface even though it may be contained completely within the cell. See Endocytosis
Eukaryotic cells are characterized by the triple-layered nature of the unit membrane. The genetic material is segregated into a central region bounded by the nuclear membrane that is penetrated by many pores containing special proteins. Bacteria (prokaryotes) do not contain such elaborate systems of internal membranes, but some have an external unit membrane separated from the plasma membrane by a special material called periplasm. The membrane does not normally flip over, so that the surface that borders the outside world, either at the cell surface or inside the cell, comes to border cytoplasm. This principle is maintained in all membranous organelles.
Mitochondria are a special case because the inner mitochondrial membrane is believed to be the membrane of a primitive one-celled organism that is symbiotically related to the cell and lies inside a cavity containing material of the outside world as defined above. The outer mitochondrial membrane is in this sense a membrane of the cell analogous to a smooth endoplasmic reticulum membrane, and the inner membrane of the mitochondrion is the plasma membrane of the included organism, which normally does not become continuous with the membrane of the cell. Thus it has its own unit membrane, and again the orientation of this unit membrane is always maintained, with one side directed toward the cytoplasm, in this case the cytoplasm of the mitochondrion. See Mitochondria
The cell membrane functions as a barrier that makes it possible for the cytoplasm to maintain a different composition from the material surrounding the cell. The unit membrane is freely permeable to water molecules but very impermeable to ions and charged molecules. It is permeable to small molecules in inverse proportion to their size but in direct proportion to their lipid solubility. It contains various pumps and channels made of specific transverse membrane proteins that allow concentration gradients to be maintained between the inside and outside of the cell. For example, there is a cation pump that actively extrudes sodium ions (Na+) from the cytoplasm and builds up a concentration of potassium ions (K+) within it. The major anions inside the cell are chlorine ions (Cl-) and negatively charged protein molecules, the latter of which cannot penetrate the membrane. The presence of the charged protein molecules leads to a buildup of electroosmotic potential across the membrane. Action potentials result from the transient opening of Na+ or calcium ion (Ca2+) channels depolarizing the membrane, followed by an opening of K+ channels leading to repolarization. This is one of the most important functions of membranes, since it makes it possible for the brain to work by sending or receiving signals sent over nerve fibers for great distances, as well as many other things. See Biopotentials and ionic currents
The plasma membrane contains numerous receptor molecules that are involved in communication with other cells and the outside world in general. These respond to antigens, hormones, and neurotransmitters in various ways. For example, thymus lymphocytes (T cells) are activated by attachment of antigens to specific proteins in the external surfaces of the T cells, an important part of the immune responses of an organism. Hormones such as epinephrine and glucagon attach to a receptor protein in the surfaces of cells and cause the activation of adenylate cyclase, which in turn causes the formation of cyclic adenosine monophosphate. Neurotransmitters attach to the postsynaptic membrane in synapses and mediate the transfer of information between neurons. There is a class of membrane proteins called cell adhesion molecules, components of the outer surfaces of cell membranes in the developing nervous system, that is thought to be involved in guiding embryonic development.
The major lipids of membranes are phospholipids with a glycerol backbone including phosphophatidyl ethanolamine, phosphatidyl choline, phosphatidyl serine, phosphatidyl inositol, and cardiolipin. Cardiolipin is more complex because it contains two glycerols and four fatty acids. It is important in bacterial membranes and is also found in the mitochondrial inner membrane.
The sphingolipids are another class of membrane lipids having the compound sphingosine as their backbone structure instead of glycerol. Ceramide is a fatty acid derivative of sphingosine that is the parent substance of many important membrane lipids. Sphingomyelin is ceramide with phosphatidyl choline added. This molecule, like phosphatidyl choline and phosphatidyl ethanolamine, is a zwitterion at pH 7; that is, it is uncharged. Phosphatidyl serine is negatively charged.
The glycolipids are an important class of lipid not containing phosphorus and based on ceramide. These include the uncharged cerebrosides that have only one sugar group, either glucose or galactose, and the gangliosides that may contain branched chains of as many as seven sugar residues including sialic acid, which is charged.
Cholesterol is a very important membrane lipid. It is present only in eukaryotes and is a prominent constituent of red blood cells, liver cells, and nerve myelin. See Cholesterol
The different lipid molecules are not equally distributed on both sides of the bilayer. The amino lipids, glycolipids, and cholesterol are located primarily in the outer monolayer, and the choline and sphingolipids are located mainly in the internal monolayer. The fatty acids of the outer half of the bilayer tend to have longer, more saturated carbon chains than those of the inner half.
The lipid bilayer has a considerable degree of fluidity, with the lipid molecules tending to rotate and translate easily, but they do not ordinarily flipflop from one side of the bilayer to the other. Furthermore, some lipids are firmly attached to membrane proteins and translate laterally only as the proteins do so. Some membrane proteins form extended two-dimensional crystals, and their lateral movement is thus restricted. Nevertheless, there is a considerable degree of fluidity in membranes overall. See Lipid
The ratio of protein to lipids in membranes is often about 1:1, but in some cases, such as nerve myelin, there is only about 20% protein. Usually polypeptide chains are folded into a globular structure with hydrophilic amino acid side chains to the outside and hydrophobic ones tucked inside. For this reason the common globular protein is hydrophilic. Sometimes stretches of hydrophobic amino acids occur in the chain and may divide it into two hydrophilic domains. If there is a stretch of hydrophobic amino acids long enough (about 20) to stretch across the hydrophobic interior of a membrane bilayer, the extrusion of the protein across the bilayer during protein synthesis may stop, leaving a hydrophilic part of the protein on the cytoplasmic side and another hydrophilic part on the outside. This protein then becomes an intrinsic amphiphilic transmembrane protein. Such proteins can be removed only with chaotropic agents that destroy the bilayer.
The classification of membrane proteins as intrinsic and extrinsic is not always easy. Some proteins clearly become attached to either the inside or outside of the bilayer by more specific interactions with the polar heads of the lipid molecules, and sometimes it is not clear whether such proteins should be called extrinsic or intrinsic. They are extrinsic in that they can be removed without using detergents to disrupt the lipid bilayer completely, but they are intrinsic in that they are permanent parts of the membrane and retain some tightly bound lipids when removed. Spectrin and anchorin in the erythrocyte membrane are firmly bound to the cytoplasmic surfaces presumably by polar head group interactions and can thus be regarded as intrinsic. See Cell (biology), Plant cell, Protein