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Lipids INTRODUCTION The lipids are a heterogeneous group of compounds. They have the common property of being (1) relatively insoluble in water and (2) soluble in nonpolar solvents such as ether, chloroform, and benzene. The lipids include fats, oils, steroids, waxes, and related compounds. Lipids are important dietary constituents not only because of their high energy value but also because of the fat-soluble vitamins and the essential fatty acids contained in the fat of natural foods. In the body fat serves as an efficient source of energy and stored in adipose tissue. It serves as a thermal insulator in the subcutaneous tissues and around vital organs. Nonpolar lipids act as electrical insulators allowing rapid propagation of depolarization waves along myelinated nerves. The fat content of nerve tissue is very high. Combinations of fat and protein (lipoproteins) are important cellular constituents, occurring in the cell membrane and in the mitochondria CLASSIFICATION Lipids are classified as simple lipid, compound lipid and derived lipids. 1. Simple Lipids: They are esters of fatty acids with various alcohols. a. Fats: They are esters of fatty acids with glycerol. A fat in the liquid state at room temperature is known as oil. b. Waxes: Esters of fatty acids with higher molecular weight monohydric alcohols. 2. Compound lipids: Compound lipids are esters of fatty acids containing groups in addition to an alcohol and a fatty acid. a. Phospholipids: Lipids containing fatty acids, an alcohol and a phosphoric acid residue. They frequently have nitrogen containing bases and other constituents. i) Glycerophospholipids : In glycerophospholipids, the alcohol is glycerol. e.g. Phosphatidyl choline, Phosphatidyl ethanolamine, Phosphatidylserine, Phosphatidyl inositol and cardiolipin ii) Glycolipids or Sphingophospholipids: In sphingophospholipids, the alcohol is sphingosine. e.g. Sphingomyeline b. Glycolipids (glcosphingolipids): Lipids containing a fatty acid, sphingosine and carbohydrate. e.g. Cerebrosides, Gangliosides c. Other compound lipids: Other compound lipids include sulfolipids, proteolipids and steroids. 3. Derived lipids: These include free fatty acids, glycerol, sterols and alcohols. Fatty aldehydes, ketone bodies, hydrocarbons, lipidsoluble vitamins, and hormone, acyglycerols (glycerides), cholesterol, and cholesteryl esters are also termed as lipids, because they are uncharged. FATTY ACIDS Fatty acids are the building block components of most lipids. Almost all naturally occurring lipids yield fatty acids upon hydrolysis. Fatty acids are aliphatic carboxylic acids. They are usually straight-chain derivatives and contain an even number of carbon atoms because they are synthesized from two-carbon units. They may be saturated containing no double bonds or unsaturated containing one or more double bonds. They occur mainly as esters in natural fats and oils and are part of other lipids. They may also occur as free fatty acids. Classification of fatty acids Fatty acids are classified into saturated and unsaturated fatty acids. i) Saturated Fatty acids: Many different kinds of saturated fatty acids have been isolated from the lipids of various compounds. Fatty acids with 16 and 18 carbons are the most abundant. Table 6.1 give some important saturated fatty acids found in naturally occurring lipids. No. of Common Carbon Name Atoms 2 Acetic acid 3 Propionic acid 4 Butyric acid 5 Valeric acid Structure Occurrence CH3COOH A major end product of carbohydrate fermentation by rumen organisms CH3CH2COOH An end product of carbohydrate fermentation by rumen organisms CH3 In certain fats, in small (CH2)2COOH amounts in milk (especially butter). An end product of CH3 (CH2)3 carbohydrate fermentation COOH by rumen organisms. 6 Caproic acid CH3 (CH2)4 COOH Present in milk fat 8 Caprylic (Octanoic) CH3 (CH2)6 COOH In small amounts in many fats (including butter), especially those of plant origin. 10 Capric acid CH3 (CH2)8 COOH Major components of many millets and seed triglycerides 12 Lauric acid CH3 (CH2)10 COOH Spermaceti, cinnamon, palm kernel, coconut oils, laurels, butter 14 Myristic acid CH3 (CH2)12 COOH Nutmeg, palm kernel, coconut oils, butter 16 Palmitic acid CH3 (CH2)14 COOH Common in all animal and plant fats 18 Stearic acid CH3 (CH2)16 COOH Ground nut (arachis)oil 20 Arachidic acid CH3 (CH2)18 COOH 22 Behenic acid CH3 (CH2)20 COOH Seed waxes 24 Lignoceric CH3 (CH2)22 COOH Cerebrosides, peanut oil and plant waxes Palmitic acid (C16) CH3- CH2- CH2- CH2-CH2-CH2- CH2- CH2- CH2CH2-CH2- CH2-CH2-CH2- CH2- COOH Stearic acid (C18) CH3- CH2- CH2- CH2- CH2- CH2- CH2- CH2- CH2CH2- CH2- CH2- CH2- CH2- CH2- CH2- CH2- COOH FATTY ACIDS Natural fatty acids may be saturated or unsaturated, and as the following data indicate, the saturated acids have higher melting points than unsaturated acids of corresponding size. The double bonds in the unsaturated compounds listed on the right are all cis (or Z). Saturated Formula CH3(CH2)10CO2H CH3(CH2)12CO2H CH3(CH2)14CO2H CH3(CH2)16CO2H CH3(CH2)18CO2H Common Name Melting Point lauric acid myristic acid palmitic acid stearic acid arachidic acid 45 ºC 55 ºC 63 ºC 69 ºC 76 ºC Unsaturated Formula Common Name CH3(CH2)5CH=CH(CH2)7CO2H -palmitoleic acid CH3(CH2)7CH=CH(CH2)7CO2H -oleic acid CH3(CH2)4CH=CHCH2CH=CH(CH2)7CO2H linoleic acid CH3CH2CH=CHCH2CH=CHCH2CH=CH(CH2)7CO2H linolenic acid CH3(CH2)4(CH=CHCH2)4(CH2)2CO2H arachidonic acid The higher melting points of the saturated fatty acids reflect the uniform rod-like shape of their molecules. The cis-double bond(s) in the unsaturated fatty acids introduce a kink in their shape, which makes it more difficut to pack their molecules together in a stable repeating array or crystalline lattice. The trans-double bond isomer of oleic acid, known as elaidic acid, has a linear shape and a melting point of 45 ºC (32 ºC higher than its cis isomer). Stearic acid & Oleic acid The shapes of stearic and oleic acids are displayed in the models below. “Essential fatty acid Two polyunsaturated fatty acids, linoleic and linolenic, are designated "essential" because their absence in the human diet has been associated with health problems, such as scaley skin, stunted growth and increased dehydration. These acids are also precursors to the prostaglandins, a family of physiologically potent lipids present in minute amounts in most body tissues.