Classification of lipids

The lipids are a group of substances found in plant and animal tissues. They are insoluble in water but soluble in common organic solvents such as benzene, ether and chloroform. They act as electron carriers, as substrate carriers in enzymic reactions, as components of biological membranes, and as sources and stores of energy. In the proximate analysis of foods they are included in the ether extract fraction.They may be classified as shown in Fig. 3.1. 

Plant lipids are of two main types structural and storage. The structiu lipids are present as constituents of various membranes and protective surface layers and make up about 7 per cent of the leaves of higher plants. The surface lipids are mainly waxes, with relatively minor contributions from long-chain hydrocarbons, fatty acids and cutin. The membrane lipids, present in mitochondria, the endoplasmic reticulmn and the plasma membranes, are mainly glycolipids (40-50 per cent) and phosphoglyc-erides. Plant storage lipids occur in fruits and seeds and are, predominantly, triacyl-glycerols. Over 300 different fatty acids have been isolated from plant tissues, but only about seven are of common occurrence. The most abundant is a-linolenic acid the most common saturated acid is palmitic acid and the most common monounsaturated acid is oleic acid. 

In animals, lipids are the major form of energy storage, mainly as fat, which may constitute up to 97 per cent of the adipose tissue of obese animals. The yield of energy from the complete oxidation of fat is about 39 MJ/kg DM compared with about 17 MJ/kg DM from glycogen, the major carbohydrate form of stored energy. In 

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Classification of Lipid-Based Delivery Systems Developed by C. Pouton (Pouton, 2000)... 

Edible oils and fats come under the biochemical classification of lipids owing to their insolubility in water. They are composed of two chemically bonded components, a fatty acid and glycerol there are many different fatty acids. When fats and oils are digested in the body, the fatty acids are liberated. Fatty acids are vital in the diet to act as starting materials for cell structures, for other important bodily chemicals including hormones and as an energy source. 

The classification of lipids by increasing efficiency can be explained by the chemical composition of the molecules (presence of polar components, hydrocarbon chain length, number of unsaturation or acetylation). For components having the same chemical nature, increasing chain length modifies the barrier properties because the polar part of the molecule decreases and does not favour water solubihty in the film (McHugh and Krochta 1994d). 

Chamberlain, L.H. (2004) Detergents as tools for the purification and classification of lipid rafts. FEBS Lett. 559,1-5. 

The classification of lipids is largely arbitrary. It can be based on water solubility (hydration) or swelling of a lipid system at the air-water interface, for example. Here, we approach lipids in terms of increasing complexity and focus on those lipids that are found mostly in cells, see Fig. 1. A more thorough discussion of the topic is given by Hauser and Poupart (1) and Larson (4). 

Several classifications of lipids are available, can be classified as follows ... 

If the classification of lipides were to be examined, some points would have to be made clear. In the first place, a definition of lipides should be made that excludes esters of mineral acids such as orthophosphoric acid. The phosphoric esters of alcohols and phenols are alkalino-stable—excluding those possessing a free carbonyl group moreover, they are not very soluble in organic solvents. The definition of lipides should therefore take into account the hydrolysis of ester linkage by heating with alkalies and the solubility of lipides in some organic solvents. 

Due to their complexity, numerous attempts have been made for further classification of lipids. The systems proposed in the literature are based on ... 

Tablcl. Classification of lipids according to their behavior to hydrolysis... 

TABLE 72—14. Classification of Lipid and Lipoprotein Levels in Adults... 

A simple classification of lipid compounds, after Deuil (1951)... 

A characteristic feature of most but not all lipases is their activation by hydrophobic-hydrophilic interfaces, whereas the activity of esterases does not depend on the presence of a lipid-water interface. However, this classification of lipid-hydrolyzing enzymes is not generally applicable. Meanwhile, lipases have been found that hydrolyse long-chain lipids but do not show interfacial activation. Thus, lipases and esterases can be better defined as enzymes hydrolizing long-chain and short-chain lipids, respectively (3)... 

Introduction and Classification of Lipids (Section 8.1) 8.1 What is the basis for deciding if a substance is a lipid 8.2 List two major functions of lipids in the human body. 8.13 From Figure 8.7, arrange the following substances in order of increasing percentage of unsaturated fatty acids chicken fat, beef fat, com oil, butter, and sunflower oil. 

Two other structures, which together with the lamellar phase are the most important liquid-crystalline phases in lipid-water systems, are shown in Fig. 8.9. It should be pointed out that the classification of lipids into polar and non-polar is best defined from their interaction with water. Lipids which do not give lipid-water phases are thus non-polar whereas those forming aqueous phases are classified as polar lipids. 

This chapter is concerned mainly with the more restrictive definition of lipids following Christie (1982). A convenient system of classification of lipids based on his scheme considers the simple lipids (compounds that on hydrolysis... 

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