Polar lipid, definition

The Definition of Lecithin. Lecithin for use in food is defined as A mixture of polar and neutral lipids with a polar lipid content of at least 60%. NB this is different from the scientific usage where lecithin is used as a trivial name for phosphatidylcholine. 

Lecithin is a polar lipid, the definition being that it is a lipid that is insoluble in acetone. It is one of a whole class of phospholipids, which tend to be found in the membranes of animals and also in plants. 

Lecithin is the widely obtained edible byproduct from oil processing. The definition of lecithin varies considerably however, lecithin represents a family of products based on naturally occurring phospholipids and other polar lipids. Most of the commercially available lecithins are obtained from various oilseeds or egg (179). In the scientihc hterature, lecithin stands for a special phospholipid, 1,2-diacyl-j n-glycero-3-phosphatidylcholine or phosphatidylchohne. 

Criteria for the identification of microemulsions have recently been published (Lindman and Danielsson, 1981). According to this definition the L2-phase in aqueous systems of polar lipids and triglyceride oil (Fig. 8.14) is a microemulsion. An X-ray study of this system and of some other ternary systems (Fontell et al., 1983) indicates... 

This chapter has given an overview of the structure and dynamics of lipid and water molecules in membrane systems, viewed with atomic resolution by molecular dynamics simulations of fully hydrated phospholipid bilayers. The calculations have permitted a detailed picture of the solvation of the lipid polar groups to be developed, and this picture has been used to elucidate the molecular origins of the dipole potential. The solvation structure has been discussed in terms of a somewhat arbitrary, but useful, definition of bound and bulk water molecules. 

By definition, the fraction that enters the circulatory system is eliminated by extrarenal mechanisms (usually metabolism by the liver and other tissues) and is derived by the difference from renal excretion that is, 1 — Fg. The excretory organs are able to eliminate polar compounds such as tetracycline and tylosin more efficiently than compounds that are highly soluble in lipids (i.e., lipophilic) such as metronidazole, erythromycin, clindamycin, and trimethoporin. Thus, the highly lipophilic compounds will not be eliminated until they are metabolized to more polar intermediates. 

Fig. 7.11. Comparison of the fine structure of the membranes of the eggs of Hymenolepis diminuta and H. nana. Note in H. nana (in contrast to H. diminuta) (a) the embryophore is thin and incomplete - a feature which may facilitate hatching in the gut of the definitive host (b) there is an additional polar filament layer between the oncospheral membrane and the basal lamina. Gc, Golgi complex ger, granular endoplasmic reticulum /, lipid bodies m, mitochondria n, nucleus v, vacuoles. (After Fairweather Threadgold, 1981a Holmes Fairweather, 1982.)...

Lipids are defined as water-insoluble compounds extracted from living organisms by weakly polar or nonpolar solvents. This definition is based on a physical property, in contrast to the definitions of proteins, carbohydrates, and nucleic acids, which are based on chemical structure. Consequently, the term lipid covers a structurally diverse group of compounds, and there is no universally accepted scheme for classifying lipids. 

Lipids are a very diverse group of biochemicals that, by definition, are poorly soluble in water and are soluble in organic solvents such as ether. An important structural feature of lipids that allows these solubility characteristics is a relatively high ratio within the molecule of hydrocarbon atoms to more polar kinds of atoms. This definition allows the inclusion of tons of molecules into the lipid family, even gasoline. We will focus on the ones normily found in the body. 

Biochemists have found it convenient to define one set of biomolecules, the lipids, as substances, insoluble in water, that can be extracted from cells by organic solvents of low polarity like ether or chloroform. This is a catch-all sort of definition, and lipids include compounds of many dilferent kinds steroids (Sec. 15.16), for example, and terpenes (Sec. 8.26). Of the lipids, we shall take up only the fats and certain closely related compounds. These are not the only important lipids— indeed, every compound in an organism seems to play an important role, if only as an unavoidable waste product of metabolism—but they are the most abundant. 

With the different definitions, different kinds of lipid classification are frequently used in the field. For example, many lipid chemists simply classify lipids into polar and nonpolar lipids based on the overall hydrophobicity of the lipids. The nonpolar lipids include fatty acids and their derivatives (e.g., long-chain alcohols and waxes), glycerol-derived lipids (e.g., monoacylglycerols (MAG), diacylglycerols (DAG),... 

The transfer of C20-C24 fatty acids (and probably of shorter fatty acyl chains) involves, at least in part, a transit through the (3olgi apparatus. To what extent, and associated with which polar heads remains to be elucidated. However, one has to keep in mind that, even if the forthcoming work definitely establishes this point, this will not imply that all lipids must use this pathway and that a cytosolic transport via LTPs is not also operative. It is thus extremely important that an in vivo approach able to study this pathway be developed, and that a visualization of the intracellular journey of the lipids destined to the plasmalemma be obtained in situ. 

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