Lipids are a class of hydrocarbon-containing organic compounds essential for the structure and function of living cells. Lipids are categorized by the fact that they are soluble in nonpolar solvents (such as ether and chloroform) and are relatively insoluble in water. Lipid molecules have these properties because they consist of mainly carbon and hydrogen, with few oxygen-containing functional groups. Some lipids are used for energy storage, others serve as structural components of cellular membranes, and some are important hormones. Although the term lipid is often used as a synonym for fat, the latter is in fact a subgroup of lipids called triglycerides. Hydrocarbons are refined at oil refineries and processed at chemical plants In chemistry, a hydrocarbon is any chemical compound that consists only of the elements carbon (C) and hydrogen (H). ... An organic compound is any of a large class of chemical compounds whose molecules contain carbon, with exception of carbides, carbonates and carbon oxides. ... In chemistry, a nonpolar compound is one that does not have concentrations of positive or negative electric charge. ... A substance is soluble in a fluid if it dissolves in the fluid. ... Ether is the general name for a class of chemical compounds which contain an ether group — an oxygen atom connected to two (substituted) alkyl groups. ... Chloroform, also known as trichloromethane and methyl trichloride, is a chemical compound with formula CHCl3. ... General Name, Symbol, Number carbon, C, 6 Chemical series nonmetals Group, Period, Block 14, 2, p Appearance black (graphite) colorless (diamond) Atomic mass 12. ... General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... General Name, Symbol, Number oxygen, O, 8 Chemical series Nonmetals, chalcogens Group, Period, Block 16, 2, p Appearance colorless (gas) very pale blue (liquid) Atomic mass 15. ... Hormone is also the NATO reporting name for the Soviet/Russian Kamov Ka-25 military helicopter. ... For other uses, see Fat (disambiguation). ... It has been suggested that Medium Chain Triglycerides be merged into this article or section. ...

Lipid functions

• Cell membrane structure
  • Constitutes a barrier for the cell
  • Controls membrane fluidity
  • Controls the flow of material in and out of the cell
• Energy storage (for instance, fats stored in adipose tissue)
• Lipid hormones like steroids and eicosanoids - mediate communication between cells
• Signal transduction - function in the transmission of information in cells
• Lipid vitamins - required for metabolism, usually as coenzymes

A hormone (from Greek horman - to set in motion) is a chemical messenger from one cell (or group of cells) to another. ... Steroid skeleton. ... In biochemistry, eicosanoids are a class of oxygenated hydrophobic molecules that largely function as autocrine and paracrine mediators. ... In biology, signal transduction is any process by which a cell converts one kind of signal or stimulus into another. ... Retinol (Vitamin A) Vitamins are nutrients required for essential metabolic reactions in the body [1]. Vitamins can act both as catalysts and participants in chemical reactions. ...

Structure

Fatty acids and glycerides

Main article: fatty acid

Chemically, fatty acids can be described as long-chain monocarboxylic acids and have a general structure of CH₃(CH₂)_nCOOH. The length of the chain usually ranges from 12 to 24, always with an even number of carbons. When the carbon chain contains no double bonds, it is called saturated. If it contains one or more such bonds, it is unsaturated. The presence of double bonds generally reduces the melting point of fatty acids. Furthermore, unsaturated fatty acids can occur either in cis or trans geometric isomers. In most naturally occurring fatty acids, the double bonds are in the cis configuration. In chemistry, especially biochemistry, a fatty acid is a carboxylic acid (or organic acid), often with a long aliphatic tail (long chains), either saturated or unsaturated. ... Covalent bonding is a form of chemical bonding characterized by the sharing of one or more pairs of electrons between atoms, in order to produce a mutual attraction, which holds the resultant molecule together. ... CIS is an abbreviation of: Commonwealth of Independent States Canadian Interuniversity Sport Contact Image Sensor CompuServe Information Service Chinese International School Confederacy of Independent Systems (Star Wars) Continuous injection system Communication Information Services Card Information Structure Cisalpino See also uses of the word cis. ... Trans is a Latin word meaning across, beyond or on the opposite side and is the opposite of cis, which means on the same side. In chemistry, a bond not subject to free rotation in which the greater radical on both ends is on the opposite side of the bond... In chemistry, geometric isomerism is a form of stereoisomerism and describes the orientation of functional groups at the ends of a bond around which no rotation is possible. ...

Glycerides are lipids possessing a glycerol (propan-1-2-30tri-ol) core structure with one or more fatty acyl groups, which are fatty acid-derived chains attached to the glycerol backbone by ester linkages. Glycerides with three acyl groups (triglycerides or neutral fats) are the main storage form of fat in animals and plants. Glycerides are esters of glycerol and fatty acids. ... Glycerol, also well known as glycerin and glycerine, and less commonly as propane-1,2,3-triol, 1,2,3-propanetriol, 1,2,3-trihydroxypropane, glyceritol, and glycyl alcohol is a colorless, odorless, hygroscopic, and sweet-tasting viscous liquid. ... General formula of a carboxylate ester. ... Triglyceride (blue: fatty acid; red: glycerol backbone) Triglycerides are glycerides in which the glycerol is esterified with three fatty acids. ...

An important type of glyceride-based molecule found in biological membranes, such as the cell's plasma membrane and the intracellular membranes of organelles, are the phosphoglycerides or glycerophospholipids. These are phospholipids that contain a glycerol core linked to two fatty acid-derived "tails" by ester or, more rarely, ether linkages and to one "head" group by a phosphate ester linkage. The head groups of the phospholipids found in biological membranes are phosphatidylcholine (lecithin), phosphatidylethanolamine, phosphatidylserine and phosphatidylinositol. These phospholipids are subject to a variety of reactions in the cell: for instance, polar head groups or fatty acid tails can be released from specific phospholipids through enzyme-catalyze hydrolysis to generate second messengers involved in signal transduction. In the case of phosphatidylinositol, the head group can be enzymatically modified by the addition of one to three phosphate molecules, and this constitutes another mechanism of cell signaling. While phospholipids are the major component of biological membranes, other non-glyceride lipid components like sphingolipids and sterols (such as cholesterol in animal cell membranes) are also found in biological membranes. A biological membrane or biomembrane is a membrane which acts as a barrier within or around a cell. ... Drawing of a cell membrane A component of every biological cell, the cell membrane (or plasma membrane) is a thin and structured bilayer of phospholipid and protein molecules that envelopes the cell. ... In cell biology, an organelle is one of several structures with specialized functions, suspended in the cytoplasm of a eukaryotic cell. ... Glycerophospholipids or phosphoglycerides are glycerol-based phospholipids. ... Two schematic representations of a phospholipid. ... Ether is the general name for a class of chemical compounds which contain an ether group — an oxygen atom connected to two (substituted) alkyl groups. ... In inorganic chemistry, a phosphate is a salt of phosphoric acid. ... A biological membrane or biomembrane is a membrane which acts as a barrier within or around a cell. ... Lecithin, also known as Phosphatidylcholine Lecithin is mostly a mixture of glycolipids, triglycerides, and phospholipids (eg Phosphatidylcholine, Phosphatidylehtanolamine, and Phosphatidylinositol). ... In biology, second messengers are low-weight diffusible molecules that are used in signal transduction to relay signals within a cell. ... In biology, signal transduction is any process by which a cell converts one kind of signal or stimulus into another. ... Cell signaling is part of a complex system of communication that governs basic cellular activities and coordinates cell actions. ... General chemical structure of sphingolipids. ... Sterols, or steroid alcohols are a subgroup of steroids with a hydroxyl group in the 3-position of the A-ring. ... Cholesterol is a sterol (a combination steroid and alcohol) and a lipid found in the cell membranes of all body tissues, and transported in the blood plasma of all animals. ...

A biological membrane is a form of lipid bilayer, as is a liposome. Formation of lipid bilayers is a spontaneous process when the glycerophospholipids described above are placed in water. In an aqueous milieu, the polar heads of lipids tend to oriente toward the polar, aqueous environment, while the hydrophobic tails tend to minimize their contact with water. The nonpolar tails of lipids (U) tend to cluster together, forming a lipid bilayer (1) or a micelle (2). The polar heads (P) face the aqueous environment. Micelles form when single-tailed amphiphilic lipids are placed in a polar milieu, while lipid bilayers form when two-tailed phospholipids are placed in a polar environment (Fig. 2). Micelles are "monolayer" spheres and can only reach a certain size, whereas bilayers can be considerably larger. They can also form tubules. Bilayers that fold back upon themselves form a hollow sphere, enclosing a separate aqueous compartment, which is the basis of biological membranes. A liposome is a spherical vesicle with a membrane composed of a phospholipid bilayer used to deliver drugs or genetic material into a cell. ... A diagonal molecular slab from the DPPC lipid bilayer simulation1; color scheme: PO4 - green, N(CH3)3 - violet, water - blue, terminal CH3 - yellow, O - red, glycol C - brown, chain C - grey. ... Schematic of a micelle. ... An amphipathic (a. ...

Micelles and bilayers separate out from the polar milieu by a process known as the "hydrophobic effect." When dissolving a nonpolar substance in a polar environment, the polar molecules (i.e. water in an aqueous solution) become more ordered around the dissolved nonpolar substance, since the polar molecules cannot form hydrogen bonds to the nonpolar molecule. Therefore, in an aqueous environment, the polar water molecules form an ordered "clathrate" cage around the dissolved nonpolar molecule. However, when the nonpolar molecules separate out from the polar liquid, the entropy (state of disorder) of the polar molecules in the liquid increases. This is essentially a form of phase separation, similar to the spontaneous separation of oil and water into two separate phases when one puts them together. Snapshot from a simulation of liquid water. ... A clathrate or clathrate compound is a chemical substance consisting of a Greek klethra, meaning bars (in the sense of a lattice). ... In thermodynamics, entropy, symbolized by S, is a state function of a thermodynamic system defined by the differential quantity , where dQ is the amount of heat absorbed in a reversible process in which the system goes from the one state to another, and T is the absolute temperature. ...

Figure 2: Self-organization of lipids. A lipid bilayer is shown on the left and a micelle on the right.

The self-organisation depends on the concentration of the lipid present in solution. Below the critical micelle concentration, the lipids form a single layer on the liquid surface and are dispersed in solution. At the first critical micelle concentration (CMC-I), the lipids organise in spherical micelles, at the second critical micelle concentration (CMC-II) into elongated pipes, and at the lamellar point (LM or CMC-III) into stacked lamellae of pipes. The CMC depends on the chemical composition, mainly on the ratio of the head area and the tail length. Lipid bilayer and micelle, from Nupedia. ... Lipid bilayer and micelle, from Nupedia. ... A diagonal molecular slab from the DPPC lipid bilayer simulation1; color scheme: PO4 - green, N(CH3)3 - violet, water - blue, terminal CH3 - yellow, O - red, glycol C - brown, chain C - grey. ... Schematic of a micelle. ... In chemistry, the critical micelle concentration (CMC) is defined as the concentration of surfactants in free solution in equilibrium with surfactants in aggregated form. ...

Nutrition and health

Lipids play diverse and important roles in nutrition and health. Many lipids are absolutely essential for life. However, there is also considerable awareness that abnormal levels of certain lipids, particularly cholesterol (in hypercholesterolemia) and, more recently, trans fatty acids, are risk factors for heart disease and other diseases. It has been suggested that Diet (nutrition) be merged into this article or section. ... Hypercholesterolemia (literally: high blood cholesterol) is the presence of high levels of cholesterol in the blood. ... A trans fatty acid (commonly shortened to trans fat) is an unsaturated fatty acid molecule that contains a trans double bond between carbon atoms, which makes the molecule less kinked compared to cis fat. Research suggests a correlation between diets high in trans fats and diseases like atherosclerosis and coronary... Heart disease is one of a number of different diseases which afflict the heart. ...

The daily requirement of lipids is 382 grams per day.

References

• Chapter 12 in "Biochemistry" by Jeremy M. Berg, John L. Tymoczko and Lubert Stryer (2002) W. H. Freeman and Co.

• Alberts, B., et al. (2004) "Essential Cell Biology, 2nd Edition." Garland Science. ISBN 0-8153-3480-X

• Solomon, Eldra P., et. al. (2005) "Biology, 7th Edition." Thomson, Brooks/Cole.

See also

• Biochemistry
• Fat
• Saturation (chemistry)

This article or section does not cite its references or sources. ... For other uses, see Fat (disambiguation). ... In chemistry, saturation has three different meanings: In physical chemistry, saturation is the point at which a solution of a substance can dissolve no more of that substance. ...

External links

• ApolloLipids - Provides dyslipidemia and cardiovascular disease prevention and treatment information as well as continuing medical education programs.
• Lipids, Membranes and Vesicle Trafficking - The Virtual Library of Biochemistry and Cell Biology
• The Lipid library - provides information on the chemistry, analysis and biochemistry of lipids
• LIPID MAPS: LIPID Metabolites and Pathways Strategy
• IUPAC glossary entry for the lipid class of molecules what is IUPAC?
• "A comprehensive classification system for lipids"
• CCMDWeb, a lipid health educational resource on global risk reduction for dyslipidemia and cardiovascular disease as well as online CME programs.

The International Union of Pure and Applied Chemistry (IUPAC) is an international non-governmental organization devoted to the advancement of chemistry. ...