cell membrane

The semipermeable membrane that encloses the cytoplasm of a cell. Also called cytomembrane, plasmalemma; Also called plasma membrane.

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 also Cell organization.

Unit membrane

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 also 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 also Mitochondria.

Function

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 also 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.

Membrane lipids

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 also 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 also Lipid.

Membrane proteins

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 also Cell (biology); Plant cell; Protein.

Every cell has a plasma membrane that encloses it and maintains differences between the cell contents and the outside environment that are crucial to the function of the cell. All biological membranes consist of assemblies of lipid and protein molecules. The lipids are rod-shaped molecules arranged in a double layer so that their hydrophobic ends, which repel water, point inwards and their hydrophilic ends, which attract it, point towards the aqueous environment and the inside of the cell. This lipid bilayer provides the basic structure of the membrane, and forms a barrier that is relatively impermeable to most water-soluble molecules. Proteins are embedded in the bilayer; they also have hydrophobic surfaces in contact with the lipids, and hydrophilic surfaces exposed on either side of the membrane. At physiological temperatures, the lipid bilayer is fluid, and so the proteins are able to move about within the plane of the membrane. The two leaflets of the bilayer contain different lipids, and different proteins are exposed on the two faces of the membrane.

The respiratory gases exchange freely across the membrane, because oxygen and carbon dioxide are soluble in lipid. Apart from this it is the proteins that span the membrane which act as pumps and channels for the exchange of materials between the inside and outside of the cell. They allow entry of nutrients into the cell and the exit of waste products. They are also responsible for generating differences in the ionic composition between the inside and the outside of the cell. Finally, proteins act as molecular sensors (membrane receptors) allowing the cell to change its behaviour in response to external chemical signals. In addition to the plasma membrane, most cells contain a variety of organelles — internal structures that are also surrounded by membranes. These include the nucleus, the endoplasmic reticulum, the Golgi complex, and the mitochondria.

Many membrane proteins are made on ribosomes (granules of nucleoprotein) bound to the membrane of the endoplasmic reticulum. Those bound for the plasma membrane are recognized and then inserted into the lipid bilayer locally, before being transported to their final destination by a trafficking system that relies on further signals within the protein. Proteins for the mitochondrial membrane are recognized and then inserted directly from the cytoplasm.

The most fundamental difference between the inside and the outside of a typical cell is in the ionic composition. In particular, the inside of the cell has a low concentration of sodium ions and a high concentration of potassium ions; the reverse is true of the fluid outside. This difference in ionic composition is generated by ion ‘pumps’, which use energy in the form of ATP, produced by mitochondrial respiration, to drive sodium ions out of the cell and potassium ions in. In addition to this ion-pumping function, most membranes contain ion channels that let ions diffuse across the membrane passively when they open. The concentration gradients for different ions across the membranes are exploited widely by cells to drive the movement of other molecules across the membrane. For example, glucose enters cells on a carrier protein that carries both sodium and glucose. Furthermore, in specialized cells, such as neurons, the ion gradients are also used to generate electrical signals that propagate along their axons and allow neurons to ‘talk’ to each other through the release of ‘neurotransmitter’ molecules.

The ability of many proteins to diffuse freely within the plane of the membrane allows them to interact transiently with protein partners, which is often crucial to their function. For instance, many receptor proteins recognize signals outside the cell and then pass the signals on to other proteins that affect cell behaviour. In other cases, though, it is important for the cell to cluster proteins at a particular region of the membrane; this is seen where receptors are localized adjacent to the site of neurotransmitter release at a synapse. This localization involves the coupling of the membrane proteins to a ‘scaffold’ within the cell, known as the cytoskeleton, via specialized anchoring proteins recruited from the cytoplasm.

Although the many organelles within the cell are enclosed by membranes, they are highly dynamic, and many are in constant communication with each other. Proteins are transported in membrane vesicles that bud from one organelle and fuse with the other; for example between the endoplasmic reticulum and the Golgi complex, and between the Golgi complex and the plasma membrane. The budding process involves the selection of proteins to be transported and the formation of a protein scaffolding that is able to pinch off a patch of membrane to form a vesicle. The vesicle must then locate and fuse with its target membrane. It is the specificity of these membrane budding-and-fusion events that permits organelles to maintain their integrity despite extensive communication between them.

A particularly good example of the specificity of membrane traffic is found in epithelial cells, such as those that form the tubules of the kidney, where the plasma membrane contains two domains that perform different functions and contain different proteins (one facing outwards to the lumen of the tubule where the urine is being formed, and the other facing inwards to the tissue fluid and the blood). The two sets of proteins are synthesized together on the membrane of the endoplasmic reticulum and are later segregated into two populations of vesicles. These vesicles are then able to recognize and fuse with the two separate domains of the plasma membrane. Without this specific targeting of proteins, epithelial polarity would break down, and epithelial secretory and absorptive function would be lost.

Cell membranes are continually in a state of flux. The delivery of new membrane into the plasma membrane is balanced by the removal of membrane by the process of endocytosis: inward budding of vesicles. Endocytosis is responsible also for the internalization of important molecules from the outside of the cell, such as cholesterol in the form of low density lipoprotein, and iron in the form of the protein transferrin. Endocytosis is also used as a route of access into the cell by rogue invaders: certain toxins, such as botulinum toxin, or enveloped viruses, such as the influenza virus.

— Michael Edwardson

See also cell; ion channels; neurotransmitters; transport.

The outer covering of a cell. The membrane controls the exchange of materials between the cell and its environment.

A selectively permeable biological membrane enveloping a cell (the cell surface membrane) or within a cell. Cell membranes consist of a bilipid layer with a variable amount of protein. They separate one structure from another and act as selective barriers, regulating the movement of materials in and out of the cell or its organelles. Conjugated proteins in cell surface membranes are involved in antigen-antibody reactions, and in distinguishing between self and non-self (usually foreign) cells.

The structure separating an animal cell from its environment or a plant cell from its cell wall. The cell membrane is a complex system that allows nutrients to enter the cell and waste products to leave, usually through osmosis.

Illustration of a Eukaryotic cell membrane

The cell membrane (also called the plasma membrane or plasmalemma) is the biological membrane separating the interior of a cell from the outside environment.^[1]

It is a semipermeable lipid bilayer found in all cells.^[2] 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 extracellular cell wall.

Contents 1 Function 2 Structure 2.1 Fluid mosaic model 2.2 Lipid bilayer 2.3 Membrane polarity 2.4 Integral membrane proteins 2.5 Membrane skeleton 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 and passed in a different form into the cell. For example, a hormone binding to a receptor could open an ion channel in the receptor and allow calcium ions to flow into the cell. 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

Fluid mosaic model

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 freely^[3]. 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. Proteins in the cell membranes may be integral or peripheral. Peripheral proteins are present on only one side of the membrane, and integral proteins span the entire membrane.

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.

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.

The arrangement of hydrophilic heads and hydrophobic tails 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 moieties 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.

Membrane polarity

Alpha intercalated cell

The apical membrane of a polarized cell is the part of the plasma membrane that forms its lumenal surface, distinct from the basolateral membrane. This is particularly evident in epithelial and endothelial cells, but also describes other polarized cells, such as neurons.

The basolateral membrane of a polarized cell is the part of the plasma membrane that forms its basal and lateral surfaces, distinct from the Apical membrane (or lumenal) surface. This is particularly evident in epithelial cells, but also describes other polarized cells, such as neurons.

"Basolateral membrane" is a compound phrase referring to the terms basal (base) membrane and lateral (side) membrane, which, especially in epithelial cells, are essentially functionally identical in composition and activity. Proteins (such as ion channels and pumps) are free to move from the basal to the lateral surface of the cell or vice versa in accordance with the fluid mosaic model.

Tight junctions that join epithelial cells near their apical surface prevent the migration of proteins to the apical membrane. The basal and lateral surfaces thus remain roughly equivalent to one another, yet distinct from the apical surface.

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. Indeed, cytoskeletal elements interact extensively and intimately with the cell membrane.^[4] 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.

Composition

Cell membranes contain a variety of biological molecules, notably 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), there is an exchange of molecules between the lipid and aqueous phases.

Lipids

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.^[5] 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 16 and 20. 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 acid chains have a profound effect on membranes fluidity^[6] as unsaturated lipids create a kink, preventing the fatty acids from packing together as tightly, thus decreasing the melting temperature (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. Under physiological conditions phospholipid molecules in the cell membrane are in the liquid crystalline state. It means the lipid molecules are free to diffuse and exhibit rapid lateral diffusion along the layer in which they are present. However, the exchange of phospholipid molecules between intracellular and extracellular leaflets of the bilayer is a very slow process. 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.^[2]

Carbohydrates

Plasma membranes also contain carbohydrates, predominantly glycoproteins, but with some glycolipids (cerebrosides and gangliosides). For the most part, no glycosylation occurs on membranes within the cell; rather generally glycosylation occurs on the extracellular surface of the plasma membrane.

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%.^[6] 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.^[5]

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, signaling, enzymatic 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 through it. Permability depends mainly on the electric charge of the molecule and to a lesser extent the molar mass of the molecule. Electrically-neutral and small molecules pass the membrane easier than charged, large ones.

The inability of charged molecules to pass through the cell membrane results in pH parturition of substances throughout the fluid compartments of the body.

See also

• Cell damage, including damage to the cell membrane
• Ammonium transporter
• AP2 adaptors
• Bacterial cell structure
• Cell adhesion
• Efflux (microbiology)
• Elasticity of cell membranes
• Gram-negative bacteria
• Gram-positive bacteria

References

1. ^ Kimball's Biology Pages, Cell Membranes
2. ^ ^{a b} Alberts B, Johnson A, Lewis J, et al. (2002). Molecular Biology of the Cell (4th ed.). ISBN 0-8153-3218-1. http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mboc4.section.1864. 
3. ^ Singer SJ, Nicolson GL (Feb 1972). "The fluid mosaic model of the structure of cell membranes". Science 175 (23): 720–31. PMID 4333397. http://www.sciencemag.org/cgi/pmidlookup?view=long&pmid=4333397. 
4. ^ Doherty GJ and McMahon HT (2008). "Mediation, Modulation and Consequences of Membrane-Cytoskeleton Interactions". Annual Review of Biophysics 37: 65-95. doi:10.1146/annurev.biophys.37.032807.125912. PMID 18573073. http://arjournals.annualreviews.org/doi/abs/10.1146/annurev.biophys.37.032807.125912. 
5. ^ ^{a b} Lodish H, Berk A, Zipursky LS, et al. (2004). Molecular Cell Biology (4th ed.). ISBN 0716731363. 
6. ^ ^{a b} Jesse Gray, Shana Groeschler, Tony Le, Zara Gonzalez (2002). "Membrane Structure" (SWF). Davidson College. http://www.bio.davidson.edu/people/macampbell/111/memb-swf/membranes.swf. 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
• Lipid composition and proteins of some eukariotic membranes

v • d • e Structures of the cell Endomembrane system Cell membrane - Nucleus (and Nucleolus) - Endoplasmic reticulum - Golgi apparatus - Peroxisome - Parenthesome Vesicles (Lysosome - Endosome - Phagosome - Vacuole) Cytoplasmic granules (Melanosome - Microbody - Glyoxysome - Weibel-Palade body) Endosymbionts Mitochondrion - Plastids (Chloroplast - Chromoplast - Leucoplast) Cytoskeleton Microfilaments - Intermediate filaments - Microtubules - Prokaryotic cytoskeleton External Cell wall - Cilium/Flagellum - Acrosome Other Cytoplasm - Ribosome - MTOCs (Centrosome/Centriole - Basal body - Spindle pole body - Vault - Proteasome

v • d • e Structures of the cell membrane Caveolae/Coated pits - Cell junctions - Glycocalyx - Lipid raft/microdomains - Membrane contact sites - Membrane nanotubes - Myelin sheath - Nodes of Ranvier - Nuclear envelope - Phycobilisomes - Porosomes

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