lipids


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lipids,

a broad class of organic products found in living systems. Most are insoluble in water but soluble in nonpolar solvents. The definition excludes the mineral oils and other petroleum products obtained from fossil material. Major classes of lipids include the fatty acidsfatty acid,
any of the organic carboxylic acids present in fats and oils as esters of glycerol. Molecular weights of fatty acids vary over a wide range. The carbon skeleton of any fatty acid is unbranched. Some fatty acids are saturated, i.e.
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, the glycerol-derived lipids (including the fats and oilsfats and oils,
group of organic substances that form an important part of the diet and also are useful in many industries. The fats are usually solid, the oils generally liquid at ordinary room temperatures.
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 and the phospholipidsphospholipid
, lipid that in its simplest form is composed of glycerol bonded to two fatty acids and a phosphate group. The resulting compound called phosphatidic acid contains a region (the fatty acid component) that is fat-soluble along with a region (the charged phosphate
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), the sphingosine-derived lipids (including the ceramides, cerebrosides, gangliosides, and sphingomyelins), the steroidssteroids,
class of lipids having a particular molecular ring structure called the cyclopentanoperhydro-phenanthrene ring system. Steroids differ from one another in the structure of various side chains and additional rings. Steroids are common in both plants and animals.
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 and their derivatives, the terpenes and their derivatives, certain aromatic compounds, and long-chain alcohols and waxeswax,
substance secreted by glands on the abdomen of the bee and known commonly as beeswax; also various substances resembling beeswax. Waxes are mixtures comprising chiefly esters of monohydroxy alcohols, besides other esters and free fatty acids, free alcohols, and higher
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. In living organisms lipids serve as the basis of cellcell,
in biology, the unit of structure and function of which all plants and animals are composed. The cell is the smallest unit in the living organism that is capable of integrating the essential life processes. There are many unicellular organisms, e.g.
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 membranes and as a form of fuel storage. Often lipids are found conjugated with proteins or carbohydrates, and the resulting substances are known as lipoproteinslipoprotein
, any organic compound that is composed of both protein and the various fatty substances classed as lipids, including fatty acids and steroids such as cholesterol.
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 and lipopolysaccharides. The fat-soluble vitaminsvitamin,
group of organic substances that are required in the diet of humans and animals for normal growth, maintenance of life, and normal reproduction. Vitamins act as catalysts; very often either the vitamins themselves are coenzymes, or they form integral parts of coenzymes.
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 can be classified as lipids. Liposomesliposome
, microscopic, fluid-filled pouch whose walls are made of layers of phospholipids identical to the phospholipids that make up cell membranes. Liposomes are used to deliver certain vaccines, enzymes, or drugs (e.g., insulin and some cancer drugs) to the body.
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 are spherical vesicles formed by mixing lipids with water or water solutions. They have found applications in the oral administration of some drugs (e.g., insulin and some cancer drugs), since they retain their integrity until they are broken down by the lipaseslipase
, any enzyme capable of degrading lipid molecules. The bulk of dietary lipids are a class called triacylglycerols and are attacked by lipases to yield simple fatty acids and glycerol, molecules which can permeate the membranes of the stomach and small intestine for use by
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 in the stomach and small intestine.

Lipids

 

fatlike substances that are found in all living cells and that play an important role in the vital processes.

Among the basic constituents of biological membranes, lipids influence cell permeability and the activity of many enzymes. They participate in the transmission of neural impulses, in muscle contraction, in the formation of intercellular contacts, and in immunochemical processes. They contribute to the formation of an energy reserve, the properties of water repellency and thermal insulation of the protective coverings of animals and plants, and the protection of the various organs against mechanical influences.

The majority of lipids are derivatives of higher fatty acids, alcohols, and aldehydes. The lipids are subdivided into several classes according to chemical composition (see Table 1). The simple lipids are substances whose molecules are composed solely of fatty-acid or aldehyde residues and of alcohols. These include fats (triglycerides and other neutral glycerides), waxes (esters of fatty acids and fatty alcohols), and diol lipids (esters of fatty acids and ethylene glycol or other dihydroxy alcohols). The complex lipids are orthophosphoric acid derivatives (phospholipids) and lipids containing sugar radicals (glycolipids). The molecules of the complex lipids also contain polyatomic alcohol radicals, such as those of glycerin (glycerin phosphatides) or sphingosine (sphingolipids). The phosphatide group includes lecithins, cephalins, polyglycerophosphatides, phosphatidyl inositol, and sphingomyelins. The glycolipids include glycosyl diglycerides, cerebrosides, and gangliosides (sphingolipids containing sialic acid radicals). Certain substances that are not fatty-acid derivatives, such as sterols, ubiquinones, and certain terpenes, are also members of the lipid group.

The chemical and physical properties of lipids are determined by the presence of both polar groupings (for example, —COOH, —OH, —NH2) and nonpolar hydrocarbon chains. It is due to

Table 1. Principal classes of lipids (R = hydrocarbon chain)
ClassFormula
Simple lipids
(1) WaxesRCOOR
(2) Diol lipidsRCOOCH2(CH2)nCH2OCOR (n = 0, 1, 2, 3)
(3) TriglyceridesLipids
Complex lipids
(1) Glycerin phosphatidesLipids
Lecithins (cholinophosphatides): Lipids 
Cephalins 
Ethanolaminophosphatides: X = CH2CH2NH2 
Serine phosphatides: Lipids 
Acid Phosphatides: X = H 
Polyglycerophosphatides 
Phosphatidyl glycerin: X = CH2CH(OH)CH2OH 
Cardiolipin: X = phosphatidyl glyceryl 
Phosphatidyl inositol: = inosityl 
(2) Glycerin glycolipidsLipids
(3) SphingoglycolipidsLipids
Cerebrosides: X = monosaccharide 
Gangliosides: X = oligosaccharide containing sialic acid radicals 
(4) SphingophospholipidsLipids
Sphingomyelins: X = Lipids 
Phytosphingolipids: X = inosityl glycoside 

this structure that the majority of lipids are surface-active, moderately soluble in nonpolar solvents (petroleum ether, benzene), and only very slightly soluble in water.

In the body, lipids are subjected to enzymatic hydrolysis by lipases. The fatty acids released are first activated by interaction with adenosine phosphoric acids (primarily ATP) and coenzyme A and then oxidized. The most common oxidative pattern consists of a series of successive detachments of two-carbon fragments, called beta oxidation. The energy liberated is used in the formation of ATP. Many lipids are present in the cells in the form of protein complexes (lipoproteins) and can be isolated only upon the destruction of the complexes (for example, with ethyl or methyl alcohol).

The investigation of the lipids extracted is usually initiated by dividing them into classes through chromatographic analysis; each class is a combination of many structurally similar substances, containing the same polar grouping but differing in fatty-acid composition. The isolated lipids are then subjected to chemical and enzymatic hydrolysis. The fatty acids released are analyzed with the aid of gas-liquid chromatography; thin-layer or paper chromatography is used to study the compounds that remain. Mass spectrometry, nuclear-magnetic resonance, and other methods of physicochemical analysis are also used to establish the structure of the products formed by lipid hydrolysis.

REFERENCES

Cherkasova, L. S., and M. F. Merezhinskii. Obmen zhirov i Iipidov. Minsk, 1961.
Markman, A. L. Khimiia Iipidov, issues 1–2. Tashkent, 1963–70.
Tiutiunnikov, B. N. Khimiia zhirov. Moscow, 1966.
Mahler, H., and E. Cordes. Osnovy biologicheskoi khimii. Moscow, 1970. (Translated from English.)
Progress in the Chemistry of Fats and Other Lipids, vols. 1–13. London, 1952–72.
Hanahan, D. J. Lipid Chemistry. New York-London, 1960.
Advances in Lipid Research, vols. 1–8. New York-London, 1963–70.
Ansell, G. B., and J. N. Hawthorne. Phospholipids: Chemistry, Metabolism, and Function. Amsterdam, 1964.
Michalec, Č. Biochemistry of Sphingolipids. Prague, 1967.

L. D. BERGEL’SON

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