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Lipids

           Lipids, including oils and fats, are a chemically diverse group of compounds that are insoluble in water and have a variety of functions. Oils and fats are the principal stored forms of energy in many organisms, with phospholipids and sterols making up approximately half the mass of biological membranes. Other crucial lipids are enzyme cofactors, electron carriers, light-absorbing pigments, hydrophobic anchers, emulsifying agents, hormones, and intracellular messengers . 

         The fatty acids found in fats and oils are highly reduced hydrocarbon derivatives whose cellular oxidation is highly exergonic. They are carboxylic acids with hydrocarbon chains of 4 to 36 carbons, which may be either fully saturated and unbranched or unsaturated with the presence of one or more double bonds. A few fatty acids contain three-carbon rings or hydroxyl groups. Some of the naturally occurring fatty acids along with their common names derived from Latin or Greek and systematic names specifying chain length and 

cake

 

number of double bonds, melting point, and solubility at 30C are shown in Table 1. A saturated lauric acid (Latin laurus, laurel plant) with 12 carbons is named 12:0 (n-dodecanoic acid), and a saturated 16-carbon palmitic acid (Greekpalma, palm tree) is named 16:0 (n-hexadecanoic acid). The 18-carbon oleic acid with one double bond

is 18: 1. The positions of double bonds are indicated with superscript numbers, e.g., a 20-carbon fatty acid with one double bond between C-9 and C-10, (C-l being the carboxyl carbon) and another between C-12 and C-13, is designated as 20:2 (L19,12). Most commonly occurring fatty acids have even numbers of carbon atoms in a linear chain of 12 to 24 carbons (Table 1). The even number of carbon atoms in these compounds reflects the mode of their synthesis, which involves condensation of 2-C acetate units. Most monounsturated fatty acids have their double bond between C-9 and C-10, the other double bonds being usually after C-12 and C-15 positions and are in the cis configuration.

The physical properties of the fatty acids and their derivatives are influenced by the length and degree of unsaturation of the hydrocarbon chain, the nonpolar hydrocarbon chain primarily acccounting for their poor solubility in water. The water solubility decreases with an increase in the length of the fatty acyl chain and a decrease in the number of double bonds. The polar carboxylic acid group accounts for the slight solubility of short-chain fatty acids in water.

           The length and degree of unsaturation of the hydrocarbon chain of fatty acids and lipids also influence their melting points (Table 1). At the ambient temperature (25C), the saturated fatty acids from C-12 to C-24 have a waxy consistency, whereas the unsaturated fatty acids of the same lengths are oily liquids. Free rotation around each C-CG bgnd in the fully saturated fatty acids enables great flexibility and more stable conformation of the fully extended hydrocarbon chain with minimum stearic hindrance from the neighboring atoms. Such molecules can pack together tightly as the result of van der Waals forces with the atoms of neighboring molecules. The presence of one or more cis double bonds interferes with this tight packing and results in less stable aggregates. The lower melting points of the unsaturated fatty acids can be attributed to the lower energy required to disorder the poorly ordered arrays of fatty acids due to the presence of double

bonds .

 

 

 

 

 

 

             

 

 

 

      

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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