phospholipid

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phospholipid

(fŏs'fōlĭp`ĭd), lipidlipids,
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.
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 that in its simplest form is composed of glycerol bonded to two 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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 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 group) that is water-soluble. Most phospholipids also have an additional chemical group bound to the phosphate. For example, it may be connected with choline; the resulting phospholipid is called phosphatidylcholine, or lecithin. Other phospholipids include phosphatidylglycerol, phosphatidylinositol, phosphatidylserine, and phosphatidylethanolamine. The bipolar character of phospholipids is essential to their biological function in cell membranes. The fat-soluble portions associate with the fat-soluble portions of other phospholipids while the water-soluble regions remain exposed to the surrounding solvent. The phospholipids of the cell membranemembrane,
structure composed mostly of lipid and protein that forms the external boundary of cells and of major structures within cells. Membrane organization is based on a sheet two molecules thick—a double layer of lipids aligned with their long hydrocarbon tails tucked
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 form into a sheet two molecules thick with the fat-soluble portions inside shielded on both sides by the water-soluble portions. This stable structure provides the cell membrane with its integrity.

Phospholipid

A lipid that contains one or more phosphate groups. Phospholipids are amphipathic in nature; that is, each molecule consists of a hydrophilic (water-loving) portion and a hydrophobic (water-hating) portion. Due to the amphipathic nature and insolubility in water, phospholipids are ideal compounds for forming the biological membrane. See Lipid

There are two classes of phospholipids: those that have a glycerol backbone and those that contain sphingosine. Both classes are present in the biological membrane. Phospholipids that contain a glycerol backbone are called phosphoglycerides (or glycerophospholipids), which are the most abundant class of phospholipid found in nature. The most abundant types of naturally occurring phosphoglyceride are phosphatidylcholine (lecithin), phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol, phosphatidylglycerol, and cardiolipin. The structural diversity within each type of phosphoglyceride is due to the variability of the chain length and degree of saturation of the fatty acid ester groups.

Sphingomyelin is the major sphingosine-containing phospholipid. Its general structure consists of a fatty acid attached to sphingosine by an amide linkage.

A bilayer membrane is formed spontaneously when phospholipids are dispersed in an aqueous solution. In this bilayer structure, phospholipids are arranged in two leaflets with the hydrophobic tails facing each other, and the hydrophilic ends exposed to the aqueous medium. Differences in the head group, the chain length, and the degree of saturation of fatty acids in the hydrophobic end are important factors in determining the shape of the bilayer. Individual phospholipid molecules are able to move freely in the lateral plane of the bilayer but not in the transverse plane (flip-flop). Small uncharged molecules are able to diffuse through the bilayer structure, but the permeability of larger or charged molecules is restricted. The arrangement of phospholipid molecules into a bilayer in an aqueous medium follows the laws of thermodynamics and represents the structural basis for the formation of all biological membranes. See Cell membranes

For a long time, phospholipids were regarded as merely building blocks for the biological membrane. It was discovered in the mid-1970s, however, that phospholipids participate in the transduction of biological signals across the membrane. For example, when the hormone vasopressin is bound to its receptor on the plasma membrane of a liver cell, the binding sets off a cascade of reactions which result in enhanced breakdown of glycogen in the liver cell, thus producing more glucose. See Second messengers

A special form of phosphoglyceride, 1-alkyl-2-acetyl-glycero-3-phosphocholine, acts as a very powerful biological mediator. It causes the aggregation and degranulation of blood platelets, and is known as platelet-activating factor (PAF).

Phospholipases are responsible for the degradation of phosphoglycerides. These enzymes are found in all tissues and in the pancreatic juice. A number of toxins and venoms have very high phospholipase activity, and several pathogenic bacteria produce phospholipases that dissolve cell membrane and allow the spread of infection. There are very few inherited diseases associated with the metabolism of phosphoglycerides; presumably, such genetic defects would be lethal during the early stage of cellular development.

Sphingomyelinase, a lysosomal enzyme, hydrolytically degrades sphingomyelin. A genetic disorder caused by a defect in the production of sphingomyelinase, called Niemann-Pick disease, leaves the cell with no or limited or ability to degrade sphingomyelin. In a severe form (type A) of this disease, the liver and spleen are sites of lipid deposits and are therefore tremendously enlarged. The lipid deposits consist primarily of the sphingomyelin that cannot be degraded. See Lipid metabolism

Phospholipid

 

(also phosphatide), any of the complex lipids containing a residue of phosphoric acid. The composition of phospholipids also includes gylcerol (or the amino alcohol sphingosine), fatty acids, aldehydes, and nitrogenous compounds (choline, ethanolamine, serine). The most important phospholipids are the phosphoglycerides (phosphatidyl choline [lecithin], phosphatidyl ethanolamine [formerly cephalin], phosphatidyl serine, phosphatidyl inositol, cardiolipin) and the sphingophospholipids, or sphingomyelins. Each class of phospholipids comprises a large number of molecules of the same type containing various fatty acids and aldehydes. The unsaturated fatty acids are usually linked to carbon atom 2 of the glycerol molecule.

Phospholipids are widely distributed in nature. As basic structural components, they form part of the cell membranes in animals, plants, and microorganisms, determining the structure and permeability of the membranes, as well as the activity of a number of enzymes within the membranes. Phospholipids react with proteins to form lipoprotein complexes. Each type of biological membrane has a characteristic makeup of phospholipids. For example, cardiolipin is characteristic of mitochondrial membranes, and sphingomyelin is found mainly in plasma membranes. The membranes of all microorganisms contain phosphatidyl glycerol and, in some cases, lecithin (unlike animal cells).

The phospholipid composition of certain organs changes with age and with the incidence of certain diseases (atherosclerosis, malignant tumors).

The separation of phospholipids and the determination of phospholipid structure are carried out with the aid of chromatography, chemical and enzymatic (using phospholipases) hydrolysis, and research techniques from physics (mass spectrometry, infrared spectrometry, nuclear magnetic resonance).

Phospholipids also include phosphonolipids, in which the phosphorus atom is bonded to a nitrogen base (choline and ethanolamine) by a covalent P—C bond. Phosphonolipids are found in a number of mollusks and bacteria.

REFERENCES

Lehninger, A. Biokhimiia. Moscow, 1974. (Translated from English.)
Form and Function of Phospholipids, 2nd ed. Amsterdam–London–New York, 1973.

E. V. DIATLOVITSKAIA

phospholipid

[¦fäs·fō′lip·əd]
(biochemistry)
Any of a class of esters of phosphoric acid containing one or two molecules of fatty acid, an alcohol, and a nitrogenous base. Also known as phosphatide.
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