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22-AUGUST-2008 06:13:22 - Cell membrane Illustration of a Eukaryotic cell membrane Illustration of a Eukaryotic cell membrane The cell membrane also called the plasma membrane, plasmalemma, or phospholipid bilayer is a selectively permeable lipid bilayer found in all cells.1 It contains a wide variety of biological molecules, primarily proteins and lipids, which are involved in a vast array of cellular processes such as cell adhesion, ion channel conductance and cell signaling. The plasma membrane also serves as the attachment point for both the intracellular cytoskeleton and, if present, the cell wall. Contents 1 Function 2 Structure 2.1 Lipid bilayer 2.2 Integral membrane proteins 2.3 Membrane skeleton 2.4 Structure and the Fluid mosaic model 3 Composition 3.1 Lipids 3.2 Carbohydrates 3.3 Proteins 4 Variation 5 Permeability 6 See also 7 References 8 External links 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, some 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 tissues. 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 that 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 that the cell can use to directly effect a response. Other proteins on the surface of the cell membrane serve as markers that 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 Diagram of the arrangement of amphipathic lipid molecules to form a lipid bilayer. The yellow polar head groups separate the grey hydrophobic tails from the aqueous cytosolic and extracellular environments. Diagram of the arrangement of amphipathic lipid molecules to form a lipid bilayer. The yellow polar head groups separate the grey hydrophobic tails from the aqueous cytosolic and extracellular environments. The cell membrane consists primarily of a thin layer of amphipathic phospholipids 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 approximately 7 nm thick, barely discernible with a transmission electron microscope.1 The arrangement of hydrophilic and hydrophobic heads of the lipid bilayer prevent polar solutes e.g. amino acids, nucleic acids, carbohydrates, proteins, and ions from diffusing across the membrane, but generally allows for the passive diffusion of hydrophobic molecules. This affords the cell the ability to control the movement of these substances via transmembrane protein complexes such as pores and gates. Flippases and Scramblases concentrate phosphatidyl serine, which carries a negative charge, on the inner membrane. Along with NANA, this creates an extra barrier to charged moities moving through the membrane. Membranes serve diverse functions in eukaryotic and prokaryotic cells. One important role is to regulate the movement of materials into and out of cells. The phospholipid bilayer structure fluid mosaic model with specific membrane proteins accounts for the selective permeability of the membrane and passive and active transport mechanisms. In addition, membranes in prokaryotes and in the mitochondria and chloroplasts of eukaryotes facilitate the synthesis of ATP through chemiosmosis. 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 membrane spanning. These may include integrins, cadherins, desmosomes, clathrin-coated pits, caveolaes, 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 that extend from the cell. Anchoring proteins restricts them to a particular cell surface - for example, the apical surface of epithelial cells 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 microtubule-based extensions covered by the cell membrane, and filopodia, which are actin-based extensions. These extensions are ensheathed in membrane and project from the surface of the cell in order to sense the external environment and/or make contact with the substrate or other cells. The apical surfaces of epithelial cells are dense with actin-based finger-like projections known as microvilli, which increase cell surface area and thereby increase the absorption rate of nutrients. Localized decoupling of the cytoskeleton and cell membrane results in formation of a bleb. Structure and the Fluid mosaic model or section by expanding it. Further information might be found on the talk page or at requests for expansion. June 2008 According to the fluid mosaic model of S. J. Singer and Garth Nicolson, the biological membranes can be considered as a two-dimensional liquid where all lipid and protein molecules diffuse more or less freely2. 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 rafts, c pickets and fences formed by the actin-based cytoskeleton; and d large stable structures, such as synapses or desmosomes. The fluid mosaic model can be seen when the membrane proteins of two cells e.g., a human cell and a mouse cell are tagged with different-coloured fluorescent labels. When the two cells are fused, the two colours intermix, indicating that the proteins are free to move in the 2D plane. 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: Fusion of intracellular vesicles with the membrane exocytosis not only excretes the contents of the vesicle but also incorporates the vesicle membrane's components into the cell membrane. The membrane may form blebs around extracellular material that pinch off to become vesicles endocytosis. If a membrane is continuous with a tubular structure made of membrane material, then material from the tube can be drawn into the membrane continuously. Although the concentration of membrane components in the aqueous phase is low stable membrane components have low solubility in water, exchange of molecules with this small reservoir is possible. 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 Examples of the major membrane phospholipids and glycolipids: phosphatidylcholine PtdCho, phosphatidylethanolamine PtdEtn, phosphatidylinositol PtdIns, phosphatidylserine PtdSer. Examples of the major membrane phospholipids and glycolipids: phosphatidylcholine PtdCho, phosphatidylethanolamine PtdEtn, phosphatidylinositol PtdIns, phosphatidylserine PtdSer. The cell membrane consists of three classes of amphipathic lipids: phospholipids, glycolipids, and steroids. The amount of each depends upon the type of cell, but in the majority of cases phospholipids are the most abundant.3 In RBC studies, 30% of the plasma membrane is lipid. 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 fluidity4 as unsaturated lipids create a kink, preventing the fatty acids from packing together as tightly, thus decreasing the melting point increasing the fluidity of the membrane. The ability of some organisms to regulate the fluidity of their cell membranes by altering lipid composition is called homeoviscous adaptation. 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 in which they are present. 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.1 Carbohydrates About 5% of the plasma membrane weight is carbohydrate, predominantly glycoprotein, but with some lipoprotein cerebrosides and gangliosides. For the most part, no glycosylation occurs on other unit membranes, and only ever occurs on the extracellular surface of cell membranes. The glycocalyx is an important feature in all cells, especially epithelia with microvilli. Recent data suggest the glycocalyx participates in cell adhesion, lymphocyte homing, and many others. The penultimate sugar is galactose and the terminal sugar is sialic acid, as the sugar backbone is modified in the golgi apparatus. Sialic acid carries a negative charge, providing an external barrier to charged particles. Proteins Type Description Examples Integral proteins or transmembrane proteins Span the membrane and have a hydrophilic cytosolic domain, which interacts with internal molecules, a hydrophobic membrane-spanning domain that anchors it within the cell membrane, and a hydrophilic extracellular domain that interacts with external molecules. The hydrophobic domain consists of one, multiple, or a combination of α-helices and β sheet protein motifs. Ion channels, proton pumps, G protein-coupled receptor Lipid anchored proteins Covalently-bound to single or multiple lipid molecules; hydrophobically insert into the cell membrane and anchor the protein. The protein itself is not in contact with the membrane. G proteins Peripheral proteins Attached to integral membrane proteins, or associated with peripheral regions of the lipid bilayer. These proteins tend to have only temporary interactions with biological membranes, and, once reacted the molecule, dissociates to carry on its work in the cytoplasm. Some enzymes, some hormones The cell membrane plays host to a large amount of protein that 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 genes in yeast code specifically for them, and this number is even higher in multicellular organisms.3 The cell membrane, being exposed to the outside environment, is an important site of cell-cell communication. As such, a large variety of protein receptors and identification proteins, such as antigens, are present on the surface of the membrane. Functions of membrane proteins can also include cell-cell contact, surface recognition, cytoskeleton contact, signalling, enzymic activity, or transporting substances across the membrane. Most membrane proteins must be inserted in some way into the membrane. For this to occur, an N-terminus signal sequence of amino acids directs proteins to the endoplasmic reticulum, which inserts the proteins into a lipid bilayer. Once inserted, the proteins is then transported to its final destination in vesicles, where the vesicle fuses with the target membrane. Variation The cell membrane has slightly different composition in different cell types and has therefore different denominations in different cell types: Sarcolemma in myocytes Oolemma in oocytes. Permeability The permeability of membranes is the ease of molecules to pass it. This depends mainly on electric charge and, to a slightly lesser extent, on the molar mass of the molecule. Electrically-neutral and small molecules pass the membrane easier than charged, large ones. The electric charge phenomenon results in pH parturition of substances throughout the fluid compartments of the body. See also Ammonium transporter AP2 adaptors Bacterial cell structure Cell adhesion Efflux microbiology Elasticity of cell membranes Gram-negative bacteria Gram-positive bacteria References ^ a b c Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell, 4th ed.. ISBN 0-8153-3218-1. ^ The fluid mosaic model of the structure of cell membranes by S. J. Singer and G. L. Nicolson in Science 1972 Volume 175, pages 720-731. ^ a b Lodish H, Berk A, Zipursky LS, et al 2004. Molecular Cell Biology, 4th ed.. ISBN 0-7167-3136-31986. ^ a b Jesse Gray, Shana Groeschler, Tony Le, Zara Gonzalez 2002. Membrane Structure SWF. Davidson College. Retrieved on 2007-01-11. External links Wikimedia Commons has media related to: Cell membrane Lipids, Membranes and Vesicle Trafficking - The Virtual Library of Biochemistry and Cell Biology Cell membrane protein extraction protocol Membrane homeostasis, tension regulation, mechanosensitive membrane exchange and membrane traffic 3D structures of proteins associated with plasma membrane of eukaryotic cells v d e Organelles of the cell Acrosome - Cell wall - Cell membrane - Centriole - Chloroplast - Cilium/Flagellum - Centrosome - Cytoplasm - Endoplasmic reticulum - Endosome - Golgi apparatus - Lysosome - Melanosome - Mitochondrion - Myofibril - Nucleus - Nucleolus - Parenthesome - Peroxisome - Plastid - Ribosome - Vacuole - Vault - Vesicle v d e Structures of the cell membrane Caveolae/Coated pits - Cell junctions - Glycocalyx - Lipid raft/microdomains - Myelin sheath - Nodes of Ranvier - Nuclear envelope - Phycobilisomes - Porosomes Retrieved from http://en..org/wiki/Cell_membrane Categories: Cell anatomy | Membrane biology | OrganellesHidden categories: Articles to be expanded since June 2008 | All articles to be expanded Views Article Discussion this page History Personal tools Log in / create account Navigation Main page Contents Featured content Current events Random article Search Go Search Interaction Community portal Recent changes Contact Donate to Help Toolbox What links here Related changes Upload file Special pages Printable version Permanent link Cite this page Languages العربية БългарÑ?ки Català Česky Dansk Deutsch Eesti Español Esperanto Ù?ارسی Français í•œêµì–´ Bahasa Indonesia Ã?slenska Italiano עברית LatvieÅ¡u Lëtzebuergesch Lietuvių МакедонÑ?ки Bahasa Melayu Nederlands 日本語 ‪Norsk bokmÃ¥l‬ Occitan Polski Português РуÑ?Ñ?кий Simple English SlovenÄ?ina SlovenÅ¡Ä?ina СрпÑ?ки / Srpski Basa Sunda Suomi Svenska ไทย Tiếng Việt Türkçe ä¸æ–‡ This page was last modified on 20 August 2008, at 15:51. of the GNU Free Documentation License. ® , Inc., a U.S.
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