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Lipids

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Lipids are non-polar (hydrophobic) organic compounds, insoluble in water, soluble in organic solvents (ether, acetone, carbontetrachloride). They contain carbon, hydrogen, and oxygen; sometimes nitrogen and phosphorus. In most cases they yield fatty acids on hydrolysis. They take place in lipid and plant metabolism. What are Lipids?

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Fatty acids Fatty acids (FAs) consist of hydrocarbon chain with a carboxylic acid at one end straight-chain organic acids Most naturally occurring fatty acids have an even number of carbon atoms They can be saturated and unsaturated. Unsaturated fatty acids have lower melting points than saturated fatty acid.

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Fatty acids Monounsaturated FAs one double bond Polyunsaturated FAs many double bonds Eicosanoids (include prostaglandins, leukotriens, prostacyclins, thromboxanes) Double bonds in fatty acids are usually in the cis configuration.

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Linoleic acid is a nutritionally essential fatty acid. that must be ingested by humans and other animals because the body requires it but cannot synthesize it. It is found in large conc. in corn, peanut, soyabean oils but not in olive oil. Absence of linoleic acid by infants weight loss and ezema. Linoleic acid

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Classification of lipids according to their structures

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Simple lipids esters of FAs (FAT, OIL, WAX) [One or two chemical identities) Hydrolysis of a simple lipid Simple lipid + H 2 O fatty acid + alcohol If the alcohol is glycerol (FAT or OIL) If the alcohol is a monohydric alcohol WAX Hydrolysis of complex lipids more fatty acids + alcohol +other compound Phospholipid + H 2 O (hydrolysis) FA + alcohol + phosphorus + nitrogen compound. Phospholipids phosphoglyceride or phosphosphingoside Glycolipid + H 2 O (hydrolysis) FA +a carbohydrate + sphingosine

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Precursor lipids compounds resulting from hydrolysis of simple or complex lipids (FA, sphingosine) Derived lipids lipids which are formed due to the transformation of fatty acids (Prostaglandins, Fat-soluble vitamins) Classification of lipids according to their structures

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Fats and oils Fats with a melting point below room temperature are called oils.

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Iodine number The iodine number of a fat or an oil is the umber of grams of iodine that will react with the double bond present in 100 g of fat or oil Higher iodine number higher degree of unsaturation Generally iodine number of animal fats < iodine number of vegetable oils Iodine number of fats 70

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Some uses of lipids in the body Fats serve as fuel. Fats serve as reserve supply of food and energy. Fats are stored in special adipose tissues and serve as a protector for vital organs. Fats act as heat insulators. Some lipids allow rapid propagation of electrical signals.

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Physical properties White or yellowish solid or liquids Pure fats and oils are odorless and tasteless. Over a period of time they become rancid and develop an unpleasant odor and tatse. Lighter than water. When shaken with water temporarily emulsion. Emulsion can made permanent by addition of emulsifying agent such soap. Fats and oils must be emulsified before they can be digested.

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Chemical reactions Hydrolysis 3. Enzyme

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Saponification

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Hydrogenation In Practice not all double bonds are hydrogenated. Hydrogenation lowers the iodine number.

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Acrolein test: Test for fats or oils which contain glycerol Glycerol (KHSO 4 ) (heat) Acrolein (strong odor) Rancidity: unpleasant odor or taste developed when fats stand at room temperature for a short period of time. Rancidity is due to hydrolysis and oxidation reactions. Oxidation of double bonds short chain aldehydes and acids bad odor and taste (Antioxidant Vit. E and C.) Fats + Water in butter (in presence of microorganisms) hydrolysis of fats Butyric acid disagreeable odor. Fats and foods containing fats have to be covered and stored in the refrigerator

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Cleansing action of soaps CH 3 -(CH 2 ) 16 -COONa (Sodium stearate) Non-Polar Polar Hydrophopic Hydrophilic Micelle Mechanical Washing causes the oil to break down into small drops Soap emulsifies then the oil and prevents it from coalescing. Soap acts also as surfactant lowers surface tension

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Detergents Synthetic compounds used as cleansing agents. Soaps do not work in hard water insoluble Ca and Mg salts Detergents work also in hard water Soaps (alkaline); Detergents (neutral) Detergents can be used on silks and wool; Sops not. Detergents: Sodium salts if long chain alcohol sulfates Example: Sodium laurylsulfate C 12 H 23 OH + H 2 SO 4 C 11 H 23 CH 2 OSO 3 H + H 2 O C 11 H 23 CH 2 OSO 3 H + NaOH C 11 H 23 CH 2 OSO 3 Na + H 2 O

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Complex lipids and cell Membranes: an overview Some functions of membranes (40-50% lipids; 50-60% Proteins) 1.Mechanical support. 2.Seperate contents of the cells from the environment 3.Structural support for proteins (pumps; receptors)

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Biochemistry II Self aggregation of lipids Introduction to lipids Polar end (hydrophilic end) non-polar end hydrophobic end)

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Glycerophospholipids Glycerophospholipids (phosphoglycerides), are common constituents of cellular membranes. They have a glycerol backbone. Hydroxyls at C1 & C2 are esterified to fatty acids. An ester forms when a hydroxyl reacts with a carboxylic acid, with loss of H 2 O.

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Phosphatidate In phosphatidate: fatty acids are esterified to hydroxyls on C1 & C2 the C3 hydroxyl is esterified to P i.

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In most glycerophospholipids (phosphoglycerides), P i is in turn esterified to OH of a polar head group ( X ): e.g., serine, choline, ethanolamine, glycerol, or inositol. The 2 fatty acids tend to be non-identical. They may differ in length and/or the presence/absence of double bonds.

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Phosphatidylinositol, with inositol as polar head group, is one glycerophospholipid. In addition to being a membrane lipid, phosphatidylinositol has roles in cell signaling.

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Phosphatidylcholine, with choline as polar head group, is another glycerophospholipid. It is a common membrane lipid.

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Each glycerophospholipid includes a polar region: glycerol, carbonyl O of fatty acids, P i, & the polar head group ( X ) non-polar hydrocarbon tails of fatty acids (R 1, R 2 ).

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Structure of phospholipids

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Sphingosine may be reversibly phosphorylated to produce the signal molecule sphingosine-1-phosphate. Other derivatives of sphingosine are commonly found as constituents of biological membranes. Sphingolipids are derivatives of the lipid sphingosine, which has a long hydrocarbon tail, and a polar domain that includes an amino group.

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