DESCRIPTION OF THE
Despite lipids are used by humans since ancient times, they were studied only through the nineteenth century by scientists probably because they are difficult to separate and identify.
No precise definition of the term lipid exists. In its broadest sense, "lipid" defines substances as oils, fats and waxes which can be only characterized by a large array of properties. They are in general
- coming from animals and plants living or fossilized,
but they can also be produced by chemical synthesis,
- insoluble or immiscible with water but soluble in organic solvents such as chloroform, ether, benzene, acetone,
- formed of long-chain hydrocarbon groups (carbon and hydrogen) but may also contain oxygen, phosphorus, nitrogen and sulfur.
This definition covers a wide range of substances which contain moieties belonging
chemically to hydrocarbons, alcohols, aldehydes, acids, amines or glucides. The majority
forms molecules such as waxes, triacylglycerols, phospholipids and glycolipids. Other
substances are also considered as lipids while belonging to other classes of molecules,
e.g. fat soluble vitamins (A, D, E and K), coenzymes (ubiquinones), pigments
(carotenoids), terpenes, phenolics and sterols. They are equally considered because they
are extracted with the cellular or membrane constituents by extraction with
fat solvents. Lipids were also defined as natural substances having in their
molecular structure one chain of at least 10 carbon atoms.
Lipids are one of the three main classes of food, besides carbohydrates and proteins. They are responsible for storing energy that animals do not immediately need.
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It can be considered that in 1786, the birth date of Chevreul ME, the chemistry of lipids was unknown. The same year, in his treatise "Eléments d'histoire naturelle et de chimie", Fourcroy AF indicated that fats ("corps gras") may be classified in pure fatty oils (olive, almond, rape), siccative oils (linseed, walnut, poppy-seed, hemp-seed) and solid oils or butter (coprah, cocoa). In 1791, Jean Senebier, a famous Swiss clergyman and scientist, reported in the Encyclopédie méthodique, Physiologie végétale (t.1, Paris 1791) a classification of oils. They are separated into essential and grassy oils. Below there is an extract of that book where the physical ans chemical properties of true oils are described.
In 1823, in his famous treatise "Recherches chimiques sur les corps gras d'origine animale", Chevreul noted that the term "corps gras" (fats) may be defined (not very different from Senebier) as compounds burning with an important flame and giving lamp-black, soluble in ethanol but not in water. After making a distinction between oils, butters, solid fats, tallow, waxes, resins, volatile oils and balms, he said: "On voit, d'après ce qui précède, combien l'expression de corps gras est vague, et l'impossibilité où l'on est d'en donner une définition scientifique" ("It can be seen from the previous description how the term "corps gras" is vague and that it appears impossible to give a scientific definition of it"). Today, the situation is not very different !!. Then, Chevreul gives what it may be considered as the first scientific classification of fats, based on acidity, volatility and the ability to be saponified. He defines two divisions and six groups. The first division (acid fats) contains long-chain and medium-chain fatty acids, the second one neutral fats) being divided into four groups : cholesterol, waxes, long-chain and medium-chain triglycerides.
Classification by Chevreul
More than one century later, in 1947, Hilditch TP in his famous book on "The chemical constitution of natural fats" (Chapman and Hall Ltd, London) claimed that "unanimity has not yet been reached in the terminology to be adopted in classifying the various types of naturally occurring compounds in which fatty acids are present ... even a collective title for the whole group is not completely settled". At that time Hilditch proposed that the term lipid should be used, as it was for the first time in 1926 (Lipid excretion III. Further studies of the quantitative relations of the fecal lipids, Sperry WM, J Biol Chem 1926, 68, 357), discarding the terms lipoids (British origin) and lipins (American origin). It is important to remind that the earliest use of the word protein was in 1868 and that of carbohydrate in 1869. Interestingly, the term "lipin" persits in "oxylipin" and "cardiolipin". Hilditch stated that the collective name "lipid" is for natural products in which the higher fatty acids are present as essential compounds. He reserved the term fats for natural triacylglycerols, solid or liquid. Lipids were organized into two sub-groups, the simple lipids, including fats and waxes (ester waxes, sterols, alcohols), and the compound or complex lipids, including phosphatides and cerebrosides.
To simplify the study of this vast group of heteroclites molecules, we propose a classification based on three major classes :
1 - Simple or neutral lipids
They contain one or two different types of compounds generally neutral except for fatty acids which are studied in this group (i.e. one sterol and/or one fatty acid, glycerol and fatty acid(s), one alcohol and/or one fatty acid, fatty acid and aminoacid...) and have non-polar properties. Some classification restricts this group to molecules containing one alcohol and/or one fatty acid. Lipids containing sugar (Glycolipids) are excluded from that list and are classified as "complex lipids", even if they contain two compounds.
2 - Complex lipids
They contain frequently three
or more chemical identities (i.e. glycerol, fatty acids and sugar, one long chain base,
one fatty acid and one phosphate group...) and have polar properties.
Some contain only two components but including a sugar moiety.
They can be divided into three main groups:
- Lipoamino acids
3 - Proteolipids (fatty
This global classification is fairly comprehensive but remains only a guide, some lipid compounds being labeled in class 1 by someone and in class 2 by others.
Another ancient classification, frequently used in the lab, is founded on the well known ability of some lipids to be hydrolyzed by basic solutions into compounds (i.e. one glycerol and one, two or three fatty acids), they are said saponifiable. The others are resistant to this type of treatment (they are unsaponifiable). There are several crossing-over between the two classifications since polar lipids can be saponifiable or not (the same for neutral lipids).
3 - Proteolipids (fatty acylated proteins)