Unlike carbohydrates, proteins and nucleic acids lipids aren’t polymers but small molecules, with a molecular weights that range between 100 and 5000, and also vary considerably in polarity, including hydrophobic molecules, like triglycerides or sterol esters, and others more water-soluble like phospholipids or very short-chain fatty acids, the latter completely miscible with water and insoluble in non polar solvents.
The little or absent water-solubility of many of them means that they are subject to special treatments at all stages of their utilization, that is in the course of digestion, absorption, transport, storage and use.
Classification of lipids
They may be classified based on their physical properties at room temperature (solid or liquid, respectively fats and oils), on polarity, or on their essentiality for humans, but the preferable classification is based on their structure.
Based on structure, they can be classified in three major groups.
- Simple lipids
They consist of two types of structural moieties.
glyceryl esters that is esters of glycerol and fatty acids: e.g. triacylglycerols, mono- and diacylglycerols;
cholesteryl esters that is esters of cholesterol and fatty acids;
waxes which are esters of long-chain alcohols and fatty acids, so including esters of vitamins A and D;
ceramides that is amides of fatty acids with long-chain di- or trihydroxy bases containing 12–22 carbon atoms in the carbon chain: e.g. sphingosine.
- Complex lipids
They consist of more than two types of structural moieties.
phospholipids that is glycerol esters of fatty acids;
phosphoric acid, and other groups containing nitrogen;
phosphatidic acid that is diacylglycerol esterified to phosphoric acid;
phosphatidylcholine that is phosphatidic acid linked to choline, also called lecithin;
phosphatidyl acylglycerol in which more than one glycerol molecule is esterified to phosphoric acid: e.g. cardiolipin and diphosphatidyl acylglycerol;
glycoglycerolipids that is 1,2-diacylglycerol joined by a glycosidic linkage through position sn-3 with a carbohydrate moiety;
gangliosides that is glycolipids that are structurally similar to ceramide polyhexoside and also contain 1-3 sialic acid residues; most contain an amino sugar in addition to the other sugars;
sphingolipids, derivatives of ceramides;
sphingomyelin that is ceramide phosphorylcholine;
cerebroside: they are ceramide monohexoside that is ceramide linked to a single sugar moiety at the terminal hydroxyl group of the base);
ceramide di- and polyhexoside that is linked respectively to a disaccharide or a tri- or oligosaccharide;
cerebroside sulfate that is ceramide monohexoside esterified to a sulfate group.
- Derived lipidsThey occur as such or are released from the other two major groups because of hydrolysis that is are the building blocks for simple and complex lipids.
fatty acids and alcohols;
fat soluble vitamins A, D, E and K;
Classification adapted from: Bloor W.R. Proc Soc Exp Biol Med, 17, 138, 1920; Christie W.W. in “Lipid Analysis” Pergamon Press, Oxford, 1982; Pomeranz Y. and Meloan C.L. in “Food Analysis; Theory and Practice” 4th ed., AVI, Westport, Connecticut, 1994; Akoh C.C. and Min D.B. “Food lipids: chemistry, nutrition, and biotechnology” 3th ed. 2008.
- They are stored in adipose tissue (triglycerides) and are one of the major energy source. Lipids are the best energy source for humans since at a parity of weight they provide the major part of calories: if carbohydrates, on average, gives 4 kcal/g, as proteins, lipids provide, on average, 9 kcal/g. Moreover, they can be present in foods without there are also fiber or water (for polysaccharides 2 g water/g) allowing to contain a great quantity of energy in a little weight.
Mostly of Nutrition Organizations recommend that lipids must contribute up to 30% (with saturated fatty acids only less than 10%) of the total daily energy intake.
- Some lipids are essential nutrients like fat-soluble vitamins A, (necessary for vision) and D (necessary for calcium metabolism), present in some fats and oils of animal origin, vitamin E (prevention of autoxidation of unsaturated lipids), present in vegetable oils, and vitamin K (normal clotting of blood) present in green leaves, essential fatty acids, in particular linoleic acid and α-linolenic acid, founders of the family of omega-6 polyunsaturated fatty acids and omega-3 polyunsaturated fatty acids, respectively.
- During growth they are utilized as “bricks” for construction of biological membranes (phospholipids, cholesterol and glycolipids together with proteins), so contributing to construction of that barrier that separates intracellular environment from extracellular one and, inside cell, circumscribes organelles like mitochondria, Golgi apparatus or nucleus, and whose integrity is the basis of life itself; moreover they are also important for maintenance, physiochemical properties and repairing of cell membranes themselves.
- Many hormones are lipids: steroid hormones, like estrogens, androgens and cortisol, are formed from cholesterol (essential also during embryogenesis), prostaglandins, prostacyclin, leukotrienes, thromboxanes, and other compounds (all eicosanids) from omega-3 and omega-6 polyunsaturated fatty acids with 20 carbon atoms.
- On plasmatic cell membranes they can act as receptors, antigens and membrane anchors for proteins and can modify the structure, and therefore the functionality, of membrane enzymes.
- Many lipids, like diacylglycerol, ceramides, sphingosine and platelet-activating factor act as regulators of intracellular processes.
- There are fat deposits not accessed during a fast, classified as structural fat, the function of which is to hold organs and nerves in the right position protecting them against traumatic injuries and shock; fat pads on the palms and buttocks protect the bones from mechanical pressure.
- A subcutaneous layer of fat is present in humans: it insulates the body reducing the loss of body heat and contributing to maintain body temperature.
- On epidermis they are involved in maintaining water barrier.
- They are electrical insulator of axon of neurons that are covered over and over again by plasmatic membranes of Swann cells, in peripheral nervous system, and of oligodendrocytes in central nervous system; these plasmatic membranes have a lipid content greater than that of the other cells. This lipoprotein coating is called myelin sheath.
- On digestive tract they facilitate the digestive process depressing gastric secretion, slowing gastric emptying and stimulating biliary and pancreatic flow.
- Bile salts (by-products of cholesterol) are natural detergents synthesized in the liver and secreted into bile. They solubilize phospholipids and cholesterol in the bile, permitting the secretion of cholesterol into the intestine (the excretion of both cholesterol and bile salts is the major way by which cholesterol is removed from the body). Bile salts also aid in the digestion and absorption of lipids and soluble-fat vitamins in gut.
- In many animals, some lipids are secreted into external environment and act as pheromones that attract or repel other organisms.
- They affect the texture and flavor of food and so its palatability. Food manufacturers use fat for its textural properties, e.g. in baked goods fat increase the tenderness of the product. Chefs know that fat addiction add to the palatability of meal and increase satiety after a meal.
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