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The cell membrane (also called the '''plasma membrane''', '''plasmalemma''' or "phospholipid bilayer") is a Semipermeable Lipid Bilayer common to all living Cells .1 It contains a wide variety of biological Molecule s, primarily Protein s and Lipids , which are involved in a vast array of cellular processes, and also serves as the attachment point for both the intracellular Cytoskeleton and, if present, the Cell Wall . FUNCTION The cell membrane surrounds the Cytoplasm of a cell and, in animal cells, physically separates the intracellular components from the extracellular environment, thereby serving a function similar to that of Skin . In Fungi , Bacteria , and plants an additional Cell Wall forms the outermost boundary, however, the cell wall plays mostly a mechanical support role rather than a role as a selective boundary. The cell membrane also plays a role in anchoring the Cytoskeleton to provide shape to the cell, and in attaching to the Extracellular Matrix to help group cells together in the formation of Tissue s. The barrier is Selectively Permeable and able to regulate what enters and exits the cell, thus facilitating the Transport of materials needed for survival. The movement of substances across the membrane can be either ''passive'', occurring without the input of cellular energy, or ''active'', requiring the cell to expend energy in moving it. The membrane also maintains the Cell Potential . Specific proteins embedded in the cell membrane can act as molecular signals which allow cells to communicate with each other. Protein Receptors are found ubiquitously and function to receive signals from both the environment and other cells. These signals are '' Transduced '' into a form which the cell can use to directly effect a response. Other proteins on the surface of the cell membrane serve as "markers" which identify a cell to other cells. The interaction of these markers with their respective receptors forms the basis of cell-cell interaction in the Immune System . STRUCTURE Lipid bilayer . The yellow Polar head groups separate the grey hydrophobic tails from the aqueous cytosolic and extracellular environments.]] The cell membrane consists of a thin layer of Amphipathic lipids which spontaneously arrange so that the hydrophobic "tail" regions are shielded from the surrounding polar fluid, causing the more hydrophilic "head" regions to associate with the cytosolic and extracellular faces of the resulting bilayer. This forms a continuous, spherical Lipid Bilayer containing the cellular components approximately 7 Nm thick, barely discernible with a Transmission Electron Microscope . The arrangement of hydrophilic and hydrophobic heads of the lipid bilayer prevents hydrophilic solutes from passively diffusing across the band of hydrophobic tail groups, allowing the cell to control the movement of these substances via Transmembrane Protein complexes such as pores and gates. Integral Membrane Proteins The cell membrane contains many Integral Membrane Proteins which pepper the entire surface. These structures, which can be visualized by Electron Microscopy or Fluorescence Microscopy , can be found on the inside of the membrane, the outside, or Through-and-through . They include Synapse s, Desmosome s, Clathrin-coated Pits , Caveolae s, and different structures involved in Cell Adhesion . Membrane skeleton The Cytoskeleton is found underlying the cell membrane in the Cytoplasm and provides a scaffolding for membrane proteins to anchor to, as well as forming organelles which extend from the cell. Anchoring proteins restricts them to a particular cell surface — for example, the ''apical surface'' of Epithelial Cell s that line the Vertebrate Gut — and limits how far they may diffuse within the bilayer. The cytoskeleton is able to form appendage-like organelles, such as Cilia , which are covered by the cell membrane and project from the surface of the cell. The apical surfaces of the aforementioned epithelial cells are dense with finger-like projections, called Microvilli , which increase cell surface area and thereby increase the absorption rate of nutrients. The cell membrane acts as a protecting body. Structure and the ''Fluid mosaic model'' According to the Fluid Mosaic Model of S. Jonathan Singer and Nicholson , the biological membranes can be considered as a two-dimensional liquid where all lipid and protein molecules diffuse more or less freely. This picture may be valid in the space scale of 10 nm. However, the plasma membranes contain different structures or domains that can be classified as (a) protein-protein complexes; (b) Lipid Raft s, (c) pickets and fences formed by the actin-based Cytoskeleton ; and (d) large stable structures, such as Synapse s or Desmosome s. COMPOSITION Cell membranes contain a variety of biological molecules, notable lipids and proteins. Material is incorporated into the membrane, or deleted from it, by a variety of mechanisms:
In all cases, the mechanical tension in the membrane has an effect on the rate of exchange. In some cells, usually having a smooth shape, the membrane tension and area are interrelated by elastic and dynamical mechanical properties, and the time-dependent interrelation is sometimes called '' Homeostasis '', ''area regulation'' or ''tension regulation''. Lipids (PtdCho), Phosphatidylethanolamine (PtdEtn), Phosphatidylinositol (PtdIns), Phosphatidylserine (PtdSer).]] The cell membrane consists of three classes of amphipathic lipids: Phospholipid s, Glycolipid s, and Steroid s. The relative composition of each depends upon the type of cell, but in the majority of cases phospholipids are the most abundant.2 The fatty chains in phospholipids and glycolipids usually contain an even number of carbon atoms, typically between 14 and 24. The 16- and 18-carbon fatty acids are the most common. Fatty acids may be saturated or unsaturated, with the configuration of the double bonds nearly always ''cis''. The length and the degree of unsaturation of fatty acids chains have a profound effect on membranes fluidity3. The entire membrane is held together via Non-covalent interaction of hydrophobic tails, however the structure is quite fluid and not fixed rigidly in place. Phospholipid molecules in the cell membrane are "fluid" in the sense that they are free to diffuse and exhibit rapid lateral diffusion along the layer they are present in. However, movement of phospholipid molecules between layers is not energetically favourable and does not occur to an appreciable extent. Lipid Rafts and Caveolae are examples of Cholesterol -enriched microdomains in the cell membrane. In animal cells cholesterol is normally found dispersed in varying degrees throughout cell membranes, in the irregular spaces between the hydrophobic tails of the membrane lipids, where it confers a stiffening and strengthening effect on the membrane. Proteins The cell membrane plays host to a large amount of protein which is responsible for its various activities. The amount of protein differs between species and according to function, however the typical amount in a cell membrane is 50%.4 These proteins are undoubtedly important to a cell: approximately a third of the Gene s in Yeast code specifically for them, and this number is even higher in multicellular organisms. The cell membrane, being exposed to the outside environment, is an important site of cell-cell communication. As such, a large variety of Protein Receptor s and identification proteins, such as Antigen s, are present on the surface of the membrane. SEE ALSO
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