This fluid lipid bilayer cross section is made up entirely of phosphatidylcholine.

A lipid bilayer or bilayer lipid membrane (BLM) is a membrane or zone of a membrane composed of lipid molecules (usually phospholipids). The lipid bilayer is a critical component of all biological membranes, including cell membranes, and so is absolutely essential for all known life on Earth. Its essential structure was discovered in 1925 by two Dutch physicians, E.Gorter and F.Grendel, while they were comparing the surface area of human erythrocytes with that of the isolated lipids in a Langmuir-Blodgett trough. They found that the area of lipids from a known number of erythrocytes, when spread out on the trough, was just twice the calculated surface area of the erythrocytes. They concluded, correctly, that the membrane is two lipid molecules thick and proposed it is made of a bilayer. Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ... Lecithin, also known as Phosphatidylcholine Lecithin is usually used as synonym for phosphatidylcholine, a phospholipid which is the major component of a phosphatide fraction which may be isolated from either egg yolk (in Greek lekithos - λεκιθος), or soy beans. ... Some common lipids. ... Phospholipid Two schematic representations of a phospholipid. ... A biological membrane or biomembrane is an enclosing or separating tissue which acts as a barrier within or around a cell. ... Look up cell membrane in Wiktionary, the free dictionary. ... Human red blood cells Red blood cells are the most common type of blood cell and are the vertebrate bodys principal means of delivering oxygen to body tissues via the blood. ... The Langmuir-Blodgett trough is a piece of laboratory equipment used to study the properties of monolayers of amphiphilic molecules. ...

Structure and function

Cross cection view of the structures that can be fored by phospholipids in aqueous solutions

The structure of a bilayer explains its function as a barrier. Lipids are amphiphilic molecules since they consist of polar head groups and non-polar fatty acid tails. The bilayer is composed of two layers of lipids arranged so that their hydrocarbon tails face one another to form an oily core held together by Van der Waals interactions, while their charged heads face the aqueous solutions on either side of the membrane. The hydrophilic interfacial regions are saturated with water, while the lipophilic core region contains essentially no water. Because of the oily core of the bilayer, it is only permeable to small hydrophobic solutes (such as chloroform or ethanol), but has a very low permeability to polar inorganic compounds and ionic molecules. For a cell, this means that even small molecules, such as sugars and salts, are contained inside it. Image File history File links Phospholipids_aqueous_solution_structures. ... Image File history File links Phospholipids_aqueous_solution_structures. ... Some common lipids. ... An amphipathic (a. ... A commonly-used example of a polar compound is water (H2O). ... A commonly-used example of a polar compound is water (H2O). ... In chemistry, especially biochemistry, a fatty acid is a carboxylic acid often with a long unbranched aliphatic tail (chain), which is either saturated or unsaturated. ... Oil refineries are key to obtaining hydrocarbons; crude oil is processed through several stages to form desirable hydrocarbons, used in fuel and other commercial products. ... Johannes Diderik van der Waals, a 1910 Nobel Prize winner, was responsible for a number of advances in physical chemistry which are named after him. ... In chemistry, hydrophobic or lipophilic species, or hydrophobes, tend to be electrically neutral and nonpolar, and thus prefer other neutral and nonpolar solvents or molecular environments. ... Look up Compound in Wiktionary, the free dictionary. ... This article is about the electrically charged particle. ...

Support for the existence of a lipid bilayer in cell membranes came with the discovery by Alec Bangham in 1965 that phospholipids, when introduced into an aqueous environment, spontaneously form liposomes. These are small balloons of lipid bilayer which can entrap polar molecules inside them. The major force driving the formation of lipid bilayers is the hydrophobic interaction between the tails and their repulsion by water. Within the interior of the membrane the hydrocarbon tails are arranged, on average, perpendicular to the plane of the membrane. The properties of the bilayer are influenced by a variety of factors, including the lipid composition, temperature and membrane pressure. The most important features are the nature of the lipid head groups and the length and degree of saturation of the hydrocarbon chains. For example, introduction of a cis-double bond into the carbon chain produces a kink which is difficult to pack with straight neighbors. This is turn leads to a more fluid hydrocarbon milieu: the more kinks there are, the greater the disorder and the more fluid the oily core becomes. Two schematic representations of a phospholipid. ... A liposome is a spherical vesicle with a membrane composed of a phospholipid bilayer used to deliver drugs or genetic material into a cell. ...

The interfacial regions of model phospholipid bilayers have a thickness of 8 to 10 Å, although they can be wider in biological membranes that include lipid molecules whose head groups have complex carbohydrates, found in the gangliosides or lipopolysaccharides ^[1]. The thickness of the hydrocarbon core region is about 27 Å for the model bilayer formed by DOPC lipid whose acyl chains have eighteen carbon atoms and a single double bond (di(C18:1)PC) ^[2]. This is close to the hydrophobic thickness of typical biological membranes measured by small angle X-ray scattering (27-32 Å) ^[3]. The thickness of a lipid bilayer increases by 0.8 Å per each additional carbon atom of the lipid tail in the liquid crystalline state, or by 1.1 Å in the gel state (these numbers should be multiplied by 2 because the bilayer has two leaflets). ^{[4] [5]}. In addition, the presence of the flat cholesterol molecule in the membrane tends to straighten out the hydrocarbon chains, causing two main effects: the membrane becomes even less permeable to small molecules and the thickness of the hydrocarbon region is increased. Phospholipid Two schematic representations of a phospholipid. ... A biological membrane or biomembrane is an enclosing or separating tissue which acts as a barrier within or around a cell. ... Ganglioside is a compound composed of a glycosphingolipid (ceramide and oligosaccharide) with one or more sialic acids (AKA n-acetylneuraminic acid) linked on the sugar chain. ... Lipopolysaccharide (captions are in French) Lipopolysaccharide (LPS) is a large molecule consisting of a lipid and a polysaccharide (carbohydrate) joined by a covalent bond. ... Lecithin, also known as Phosphatidylcholine Lecithin is usually used as synonym for phosphatidylcholine, a phospholipid which is the major component of a phosphatide fraction which may be isolated from either egg yolk (in Greek lekithos - λεκιθος), or soy beans. ... Some common lipids. ... A biological membrane or biomembrane is a membrane which acts as a barrier within or around a cell. ... SAXS (small-angle X-ray scattering) is a small-angle scattering (SAS) technique where the scattering of X-rays (wavelength 0. ... Schlieren texture of Liquid Crystal nematic phase Liquid crystals are substances that exhibit a phase of matter that has properties between those of a conventional liquid, and those of a solid crystal. ... Cholesterol is a sterol (a combination steroid and alcohol). ...

The boundary between the hydrocarbon core region and the water-saturated interface is very narrow (~3Å) and defined by the effective concentration of water ^[6] that changes exponentially from nearly zero to ~2M ^[7]. The hydrocarbon boundary plane passes through the carbonyl groups of phospholipids. The phosphate groups of phospholipids are completely hydrated and situated ~5 Å outside the hydrophobic membrane boundaries. ^[8] Phospholipid Two schematic representations of a phospholipid. ... Phospholipid Two schematic representations of a phospholipid. ...

Lipid bilayers have certain elastic properties. Free bilayers have zero surface tension and do not support shear stress, like liquids. However, they have non-zero Young's modulus and bulk modulus. They also can be described as having an internal lateral pressure or "the sponaneous radius of curvature", which defines the tendency of the lipid molecules to form curved rather than planar surfaces. ^[9] An elastic modulus, or modulus of elasticity, is the mathematical description of an object or substances tendency to be deformed when a force is applied to it. ... Surface tension is an effect within the surface layer of a liquid that causes that layer to behave as an elastic sheet. ... Shear stress is a stress state where the stress is parallel or tangential to a face of the material, as opposed to normal stress when the stress is perpendicular to the face. ... For other uses, see Liquid (disambiguation). ... In solid mechanics, Youngs modulus (E) is a measure of the stiffness of a given material. ... The bulk modulus (K) of a substance essentially measures the substances resistance to uniform compression. ...

Bilayers in cell membranes and lipid asymmetry

The membranes in cells are more complicated because they contain a variety of different classes of lipids, although the vast majority of the lipids are phospholipids and, in the case of higher cells, cholesterol. The main phospholipids are phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol and sphingomyelin. Glycolipids usually account for a few percent of the lipid molecules. Cholesterol is a sterol (a combination steroid and alcohol). ... Lecithin, also known as Phosphatidylcholine Lecithin is usually used as synonym for phosphatidylcholine, a phospholipid which is the major component of a phosphatide fraction which may be isolated from either egg yolk (in Greek lekithos - λεκιθος), or soy beans. ... Phosphatidylethanolamine is a lipid found in biological membranes. ... Phosphatidylserine is a phospholipid nutrient found in fish, green leafy vegetables, soybeans and rice, and is essential for the normal functioning of neuronal cell membranes. ... Chemical structure of sn-1-stearoyl-2-arachidonoyl phosphatidylinositol Phosphatidylinositol (abbreviated PtdIns, or PI) is a minor phospholipid component of eukaryotic cell membranes. ... Sphingomyelin is a type of sphingolipid found in animal cell membranes, especially in the membranous myelin sheath which surrounds some nerve cell axons. ... Glycolipids are carbohydrate-attached lipids. ...

In mammalian cells, the phospholipids are not uniformly distributed between the two monolayers. As discovered in the human erythrocyte by M. Bretscher in 1972, the outer monolayer is largely composed of phosphatidyl-choline and sphingomyelin whereas the cytoplasmic monolayer contains the phosphatidyl-ethanolamine, phosphatidyl-serine and phosphatidyl-inositol. In addition, glycolipids are always found in the outer, non-cytoplasmic side. These asymmetries give the cell's bilayer properties different to those found in a liposome. For example, the negatively charged phospholipids, phosphatidyl-serine and phosphatidyl-inositol are found solely on the cytoplasmic side: this imparts a strong charge gradient across the bilayer which must affect the properties of transmembrane proteins and enzymes. The existence of lipid asymmetry in membranes depends on the very slow rate at which individual phospholipids can “flip-flop” — spontaneously move from one monolayer to the other — as first documented by R. Kornberg and H. McConnell in 1971. By contrast, the uncharged cholesterol readily flip-flops between monolayers and is approximately equally distributed on each side of the bilayer. MARK BRETSCHER (1940- ), biological scientist and Fellow of the Royal Society Mark Bretscher joined Abingdon School in 1951 and entered Gonville and Caius College, Cambridge in 1958. ... Sphingomyelin is a type of sphingolipid found in animal cell membranes, especially in the membranous myelin sheath which surrounds some nerve cell axons. ... Roger D. Kornberg Roger David Kornberg (born April 24, 1947) is an American scientist and professor of structural biology at Stanford University School of Medicine. ... Harden M. McConnell (1927-) is an American physical chemist at Stanford University[1]. // Harden M. McConnell was born on July 18, 1927 in Richmond, Virginia. ...

The asymmetry of phospholipids reflects their biosynthetic pathway. The four phosphatidyl-containing lipids are all synthesised on the cytoplasmic side of the endoplasmic reticulum, an internal membrane. The phosphatidylcholine, but not the other phospholipids, is then transported to the lumenal, non-cytoplasmic side, of that membrane by a flippase; this generates an asymmetry. Sphingomyelin is synthesised on, and incorporated into, the non-cytoplasmic side of the bilayer in the Golgi apparatus. The endoplasmic reticulum or ER is an organelle found in all eukaryotic cells that is an interconnected network of tubules, vesicles and cisternae that is responsible for several specialized functions: Protein translation, folding, and transport of proteins to be used in the cell membrane (e. ... Flippases (rarely, flipases) are enzymes located in the membrane responsible for aiding the movement of phospholipid molecules between the two leaflets that compose a cells membrane (transverse diffusion). ... Micrograph of Golgi apparatus, visible as a stack of semicircular black rings near the bottom. ...

The asymmetry of the lipid bilayer has important biological functions. Thus, phosphatidylinositol and its phosphorylated derivatives are key signalling molecules inside the cell. Phosphatidylserine is used to indicate to scavenging macrophages that a dying cell should be removed: during apoptosis a phosphatidyl-serine specific flippase or scramblase is activated, revealing this lipid on the surface of the dying cell. This is recognised by macrophages which consume the corpse. A section of mouse liver showing an apoptotic cell indicated by an arrow Apoptosis (pronounced apo tō sis) is a process of suicide by a cell in a multicellular organism. ... Flippases (rarely, flipases) are enzymes located in the membrane responsible for aiding the movement of phospholipid molecules between the two leaflets that compose a cells membrane (transverse diffusion). ... To meet Wikipedias quality standards, this article or section may require cleanup. ...

Bacteria also appear to have asymmetric bilayers, although the evidence is less clear-cut. The main lipids in many bacteria are phosphatidylethanolamine, phosphatidylglycerol and cardiolipin. In Bacillus megaterium, there appears to be about twice as much phosphatidylethanolamine in the cytoplasmic monolayer as in the outer one. This implies that much of the phosphatidylglycerol and cardiolipin may be in the outer monolayer. Phosphatidylethanolamine is a lipid found in biological membranes. ... Phosphatidylglycerol is a glycerophospholipid found in pulmonary surfactant. ... Cardiolipin Cardiolipin (alternate image) Cardiolipin (bisphosphatidyl glycerol) is an important component of the inner mitochondrial membrane, where it constitutes about 20% of the total lipid. ... Binomial name Bacillus megaterium Bacillus megaterium is a rod-shaped, Gram-positive species of bacteria used as a soil inoculant in agriculture and horticulture. ...

Model lipid bilayers

Within a critical concentration range, certain lipids will self-organize in water to form a bilayer. Such membranes can be used in research, for example electrical experiments on the bilayer by using the patch clamp technique. Self-organization refers to a process in which the internal organization of a system, normally an open system, increases automatically without being guided or managed by an outside source. ... A bilayer is a closely packed double layer of atoms or molecules. ... Classical patch clamp setup, with microscope, antivibration table and micro manipulators Complete miniaturized planar patch clamp setup Patch clamp technique is a technique in electrophysiology that allows the study of individual ion channels in cells. ...

Note that membrane-spanning proteins are erronously referred to as globular proteins in the opposite figure. Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ...

Black BLM: a BLM over an aperture between two aqueous solutions. The advantage of this method is the ability to control the constituents of each side of the membrane. The disadvantage of this method is that it causes the membrane to be fairly unstable, and rupture is certain in a matter of hours.

Supported BLM (s-BLM): a BLM covering an electrode patterned on a substrate. This method has the advantage of producing a stable membrane, which in some cases may last several days before rupturing. The drawback of this method is that it is only possible to control the solution on the side of the membrane that is not in contact with the electrode, although studies show that a 1 nanometre-thick water layer forms between the membrane and the electrode, this is of insufficient volume for controlling the solution composition.

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Other lipid structures

Lipids can assume self-organized structures other than bilayers, depending on their concentration, chemical structure, and experimental conditions: micelles, monolayers, or different lipid mesophases. See lipid polymorphism Self-organization refers to a process in which the internal organization of a system, normally an open system, increases automatically without being guided or managed by an outside source. ... Schematic of a micelle. ... A monolayer is a single, closely packed layer of atoms or molecules [1]. A Langmuir monolayer is a one-molecule thick insoluble layer of an organic material spread onto an aqueous subphase. ... Polymorphism in biophysics is the aspect of the behaviour of lipids that influences their long-range order, i. ...

References

1. ^ McIntosh T.J, Vidal A., and Simon S.A. 2002. The energetics of peptide-lipid interactions: modification by interfacial dipoles and cholesterol. In Current Topics in Membranes 52: 205-253.
2. ^ Lee, A.G. 2003 Lipid-protein interactions in biological membranes: a structural perspective. Biochim. Biophys. Acta 1612: 1-40.
3. ^ Mitra, K., Ubarretxena-Belandia, Tim O'conner., Taguchi, T., Warren, G., and Engelman, D.M. 2004. Modulation of the bilayer thickness of exocytic pathway membranes by membrane proteins rather than cholesterol. Proc. Natl. Acad. Sci. 101: 4083–4088.
4. ^ Lewis, B.A. and Engelman, D.M. 1983a. Lipid bilayer thickness varies linearly with acyl chain length in fluid phosphatidylcholine vesicles. J. Mol. Biol. 166: 211–217.
5. ^ Dumas, F., Lebrun, M.C., and Tocanne, J.F. 1999. Is the protein/lipid hydrophobic matching principle relevant to membrane organization and functions? FEBS Lett. 458: 271–277.
6. ^ Marsh, D. 2001. Polarity and permeation profiles in lipid membranes. Proc. Natl. Acad. Sci. 98: 7777–7782.
7. ^ Marsh, D. 2002. Membrane water-penetration profiles from spin labels. Eur. Biophys. J. 31: 559–562.
8. ^ Nagle, J.F. and Tristram-Nagle, S. 2000. Structure of lipid bilayers. Biochim. Biophys. Acta 1469: 159–195.
9. ^ McIntosh T.J. and Simon S.A. 2006. Roles of bilayer material properties in function and distribution of membrane proteins. Annu. Rev. Biophys. Biomol. Struct. 35: 177-198.

External links

• Structure of Fluid Lipid Bilayers, from the Stephen White laboratory at University of California, Irvine

The University of California, Irvine is a public research university primarily situated in suburban Irvine, California, USA. Founded in 1965, it is one of ten University of California campuses and is commonly known as UCI or UC Irvine. ...

See also

• Biological membrane
• Vesicles
• Liposomes
• Elasticity of cell membranes
• Extracellular matrix
• Phospholipid
• Category: surfactants
• Protobiont