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CHAPTER – III LIPIDS Introduction, occurrence, classification, biochemical functions and physiological importance of lipids. INTRODUCTION Fats and their derivatives are known collectively as lipids. (Greek word: lipos= fat) The principal component of lipid is fatty acid. The term lipid was first time given by biochemist BLOOR in 1943. Bloor defined lipid as ` Naturally occurring compound which are insoluble in water and soluble in organic solvents such as benzene, chloroform, ether etc. On hydrolysis they yield fatty acids. OCCURRENCE They are widely distributed throughout plant and animal kingdom. In plants, they are present in seeds, nuts and fruits while they are present in adipose tissues in animals, bone marrow and nervous tissues. CLASSIFICATION The lipids are classified in THREE groups. 1. Simple lipids : Fats , oils , wax 2. Compound lipids : Phospholipids, glycolipids, gangliosides, lipoproteins 3. Steroids: Cholesterols, ergosterols, sex hormones. 1 SIMPLE LIPID: Simple lipids are the esters of fatty acids with various long chain alcohols. Fatty acids are of two types. 1. Saturated Fatty acids. 2. Unsaturated fatty acids. Saturated Fatty acids. Butyric acid Caproic acid Caprylic acid Lauric acid Myristic acid Palmitic acid Stearic acid Arachidic acid CH3CH2CH2COOH CH3 (CH2) 4COOH CH3 (CH2)6 COOH CH3 (CH2)10 COOH CH3 (CH2)12 COOH CH3 (CH2)14 COOH CH3 (CH2)16 COOH CH3 (CH2)18 COOH Butter Butter Coconut, palm oil Laurel.kernel oil Nutmeg oil Palm oil Cocoa butter Peanut oil. Unsaturated fatty acids : 1. Oleic acid (C18 ) 2. Linoleic acid (C18 ) 3. Linolenic acid (C18 ) 4. Erucic acid (C22 ) 5. Arachidonic acid (C20 ) One double bond Two double bond Three double bond One double bond One double bond Olive oil Linseedoil,Soybean oil Linseed oil Mustard oil Ground nut oil 43 No. (2), (3) and (5) are the essential unsaturated fatty acids required by the human being. CLASSIFICATION OF LIPIDS Lipids Simple lipid Compound Lipid Sterols Fat Oil Wax Cholesterol Carotenoids Ergosterol Sex hormone Phosho Glyco Lipid lipid Lecithin Cerebroside Cephalin Hydrocarbon Vitamin A, D,E & K Ganglioside Lipo | Protein N-acetyl Protein+ Neuramic triglyceride Acid + + cholesterol Fattyacid + or phospho hexose lipid COMPARISON OF FATS AND OILS: FAT OILS 1. Esters of fatty acids + glycerol 1. Esters of fatty acids + glycerol 2. Solid at room temperature 2. Liquid at room temperature 3. Low molecular weight 3. High molecular weight 4. More saturated fatty acids 4. Higher unsaturated fatty acids 5. High saponification value 5. Low saponification value 6. Low iodine value 6. High iodine value CHEMICAL PROPERTIES OF SIMPLE LIPIDS: 1. HYDROLYSIS: They are hydrolysed by super heated steam, strong alkali, dilute mineral acid or lipase enzyme. Lipids + H2O ------------------------- Fatty acid Lipase enzyme + glycerol 44 2. SAPONIFICATION: when fat or oil reacts with NaOH or KOH, they yield salts of fatty acid (soap) and glycerol. Neutral fat + NaOH/KOH ------ Soap + glycerol 3. HYDROGENATION: Unsaturated fatty acids react with hydrogen in presence of nickel at 150-190 0 C to give saturated fatty acids. Liquid oil is converted to solid fat. Hydrogenation is the process used for preparing the artificial ghee (vegetable oil). Unsaturated fatty acid + H2 --------- Saturated fatty acid CH3 (CH2)7 CH = CH CH2COOH CH3 (CH2)7 CH2 CH2CH2COOH 4. HALOGENATION: When unsaturated fat is treated with halogens (I2 / Br2 /Cl2 ), the fat is converted to halogenated compounds. E.g. Iodine value is a useful tool to know the amount of unsaturation in the fat. CH3 (CH2)7 CH= CH CH2COOH + I2 / Br2 /Cl2 -- CH3 (CH2)7 CH - CH CH2COOH I/Br/Cl I/Br/Cl Halogenated compound 5. RANCIDITY: When butter/oil/fat is stored for a long time, they often become rancid due to hydrolysis/oxidation. The rancid fat gives off flavour, bad smell and unpleasant taste. The rancidity can be defined as the production of bad unpleasant smell in oil or fat is called rancidity. The rancidity is of two types. (A) Hydrolytic rancidity (B) Oxidative rancidity Aldehydic rancidity Ketonic rancidity A) Hydrolytic rancidity: This is developed by the action of microbial lipase enzyme which split the triglycerides and produce low molecular weight (C 4 – C10 ) fatty acid such as butyric acid. This butyric acid produces bad smell in the butter. Butter or butterfat + Lipase + 3H20 ----- Glycerol + Butyric acid B) Oxidative rancidity: This type of rancidity develops due to oxidation of unsaturated fats to produce either the cleavage or polymerisation of fatty acid. Two types of oxidative rancidity a) Aldehydic rancidity b) Ketonic rancidity 45 1) Aldehydic rancidity: There is production of peroxides when these peroxides are converted into aldehyde which gives bad smell in fat. O -- O HC = CH + O2 --- - C - C ------ H Unsaturated fatty acid H Peroxide C=O H Aldehyde 2) Ketonic rancidity: When ketones are formed from the β oxidation of saturated fatty acid, then keto acids are formed. Elimination of CO 2 leads to rancid ketone formation to produce bad smell. CH3 (CH2) n COOH -- Saturated fatty acid CH3 CH2- CH2 – C – COOH || Keto acid O CH3CH2CH2COCH3 Ketone (bad smell) CLASSIFICATION OF OILS ON THE BASIS OF THEIR DRYING CHARACTERS OILS DRYING OILS Oxidation of unsaturated Fatty Acids lead to tough Water-Proof film e.g linseed oil, oil paint SEMIDRYING OILS NON DRYING OILS Dries slowly in presence of oxygen Remain unaffected by Presence of oxygen e.g. Oleic acid, Linoleic acid e.g. greasing oil Present in cotton seed for lubrication. And soybean QUALITATIVE TESTS (ANALYTICAL CONSTANTS) TO DETERMINE PURITY OF OIL / FAT: It is very essential to keep the oil in the pure, hygienic form. Upon storage or adulteration (malpractice by the oil millers), there is a chance of adulteration in the purity of oils. Analytical constants help in judging the quality of oil. 46 1. SAPONIFICATION VALUE: It can be defined as milligrams of NaOH / KOH required saponifying one gram of oil or fat is called saponification value. This test suggests the average chain length of fatty acids in the oil. 2. ACID VALUE: It can be defined as the milligram of KOH to neutralise the FREE FATTY ACIDS present in one gram of oil/fat. This test suggests us the amount of free fatty acids. When the fat becomes rancid due to hydrolysis free fatty acids are produced. 3. IODINE NUMBER: It can be defined as number of centi grams (1 centigram = 10 mg.) of iodine taken up by one gram of oil. Iodine value suggests the amount of unsaturated fatty acids present in the oil. More iodine value = better the oil for consumption. 4. REICHERT- MEISSL NUMBER (R.M.Number): It can be defined as the number of ml of 0.1 N alkali required to neutralise WATER SOLUBLE and volatile fatty acid contained in 5 grams of oil / fat. If the RM value decrease / increase beyond the range, there is a chance of adulteration. 5. POLENSKE NUMBER (P NUMBER): It can be defined as the number of ml of 0.1 N alkali required to neutralise WATER INSOLUBLE and volatile fatty acid contained in 5 grams of oil / fat. If the P value decrease / increase beyond the range, there is a chance of adulteration. 6. KRISCHNER VALUE (K VLUE): It can be defined as the number of ml of 0.1 N alkali required to neutralise WATER SOLUBLE and volatile fatty acid contained in 5 grams of oil / fat. If the K value decrease / increase beyond the range, there is a chance of adulteration. The quantity of butyric acid can be measured by this test. IMPORTANCE OF GLYCERIDES IN EVERYDAY LIFE: Manufacturing vegetable ghee (Hydrogenation) Soap Industry (Saponification) In Paint and Varnish Industry (Drying oils) Candle Industry ( Wax) Medicine (Castor oil, coconut oil, cod liver oil) Cosmetics Industry ( Toilet soap, nail polish, perfumes) Lubricant Industry ( Greasing oil, castor oil) 47 WAX These are the esters of aliphatic monohydric alcohol with higher fatty acids. E.g. Bee wax ----------- Neoceryl alcohol Carnauba wax------- Carnauba plant Spermacetic Wax -- Sperm whale n- octacosanol ----- Wax of wheat blade Waxes are also found in plants (particularly aquatic) and microorgs where they form a protective covering (in leaves and fruit) and also found in insect secretions. COMPOUND LIPIDS They are the complex esters of aliphatic mono carboxylic fatty acid with aliphatic alcohols having nitrogenous base such as choline, ethanolamine etc. 1. PHOSPHOLIPIDS: These are the esters produced by the combination of fatty acid, glycerol, phosphoric acid and a nitrogenous base. E.g. Lecithin, Cephalin, Plasmlogens, Phosphoinositis, Sphingomyelins LECITHIN: They are found in the growing tissues, egg yolk, brain, kidney, soybean, nervous tissues, blood etc. They are occurring in two isomeric forms. a) α - Lecithin and b) β – Lecithin STRUCTURE OF α - LECITHIN CH2 CO – R’ (STEARIC ACID) | | CHO CO – R” (OLEIC ACID) | O CH3 | CH2O –O – P – O ---- CH2-CH2—N – CH3 OH Phosphoric Acid OH CH3 Choline residue STRUCTURE OF β - LECITHIN 48 CH2 CO – R’ (STEARIC ACID) CH3 | O | CHO - ---- O – P – O ---- CH2-CH2—N – CH3 | | OH OH CH3 | | CH2O –CO – R” (OLEIC ACID) FUNCTIONS OF LECITHIN Controls fat metabolism Used as food product in the form of emulsifying agent Acts as anti-oxidant to prevent rancidity Regulate the permeability of cell membrane CEPHALIN: The difference between lecithin and cephalin is only in the structure. Choline residue is replaced with the cholamine, attached to glycerol. Their main function is in nervous tissues. Plasmalogen, phosphoinositides and sphingomyelins are components in the human tissues and have little importance in the plants. 2. GLYCOLIPID: It contains carbohydrate moiety particularly galactose in the structure. E.g. Cerebroside is present in brain tissues and act as neuro transmitters. STEROLS Sterols are the solid alcohols with high molecular weight having many cyclic rings in the structure. ( Steros = solid ) , ( ol = alcohol ). They are widely distributed in plant, animals and micro organisms in conjugated form. 1. CHOLESTEROL : It is a animal sterol with a molecular formula of C27H45 OH 2. ERGOSTEROL: It is a mycosterol with a molecular formula C28H43 OH. 3. Sistosterols or Stigmsterols : It is a phytosterol of plant origin. 49 METABOLISM OF LIPIDS: 1. ANABOLISM: Glucose Glucose Glycolysis Glycolysis Glyceraldehyde Pyruvuc acid Glycerol Acetyl Co –A Fatty acid Glycerol + synthesis Fatty acid -----------> Lipid 2. CATABOLISM: Lipid -------------> Fatty acid + Glycerol Lipase β –oxidation Acetyl co-A Glycolysis Pyruvic acid ------> Acetyl co -A TCA cycle TCA cycle CO2 + H2O + ENERGY 50 PHYSIOLOGICAL FUNCTIONS AND IMPORTANCE OF LIPIDS: 1. The main function of fat is that it serves as a source of energy. One gram of fat gives 9 Kcal. Excess foodstuff is converted to fat in the adipose tissues in the human. 2. They are structural components of cells. Phospholipids and sterols are present in biological membrane. They control the movement of materials into and out of the cell. Lipo proteins also play important role. 3. It acts as a carrier of essential compounds such as fat soluble vitamins (A, D, E and K). Lipid act as solvent in the body fluid. 4. Their main function is to provide essential fatty acids such as Linoleic acid, Linolenic acid and Arachidonic acid. 5. Enzyme activation: They sometimes act as enzyme activator. E.g. Phosphatase requires presence of lipid for maximum activity. 6. Synthesis of hormone such as adrenal corticoid, sex hormones and vitamin D3 (Cholecalciferol) from lipid derivatives. 7. Heat Insulator: Presence of fats in subcutaneous tissues provide heat insulation e.g. Whale. It also provides heat insulation to animals of Antarctic regions and animals living in cold environments. 8. Mechanical Protection: It provides mechanical protection to vital organs such as heart, brain etc. Because of the presence of waxy materials, it protects the surface from the attack of microorganisms and insects. 9. Terpenes have medicinal use as mild local irritant. 10. They are used in the perfume and flavour industry because of less solubility in water. 11. It reduces the bulk of the diet. 12. Some essential oils make the food unpalatable to animals while some aroma is released to attract the insect for the cross-pollination 51