Other Fatty Biological Molecules

Triglycerides. Fat is stored in the body in the form of triglycerides and can also be present in blood plasma. These lipid molecules are insoluble in water and consist of a glycerol with three fatty acid tails. There are many different forms of triglyceride, and each of the fatty acid tails on the molecule is usually different, varying in length and degree of unsaturation. 

Waxes are a variation of oil molecules, are the solid form of hydrocarbon alkane chains more than about 20 carbons long, and are insoluble in water. These materials are usually amorphous plastic solids at room temperature, very hydrophobic, and can be found in many plants and organisms acting as a waterproof coating. Some natural waxes include beeswax, lanolin, and carnauba. Paraffin wax is typically derived from crude oil for commercial use, although many plant waxes are very similar in composition, a mixture of mostly straight-chain hydrocarbons. 

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Lipids occur widely in nature. For examples, phospholipids are the main building blocks of biological membranes. Lipids exist in a variety of distinct forms, such as fatty acids, acylglycerols, phospholipids, sphingomyelins, glycosphingolipids, gangliosides, steroids, bile acids, prostaglandins, and leukotrienes. Some of lipids are structurally simple molecules, but others are complex molecules. Many are amphiphilic compounds. 

The functional groups of acyl substitution reactions all relate to carboxylic acids. They include acyl chlorides, anhydrides, esters, amides, thioesters, carboxylic acids themselves, and others that we shall study in this chapter. In Special Topic C we shall see how acyl substitution reactions are used to synthesize polymers such as nylon and Mylar. In Special Topic E we shall consider the biosynthesis of fatty acids and other biological molecules by acyl substitution reactions. Although many functional groups participate in acyl substitution reactions, their reactions are all readily understandable because of the common mechanistic theme that unites them nucleophilic addition-elimination at an acyl carbon. 

In comparison, addition of organic molecules such as methanol or ethanol decreases the magnitude of y of water from 72 mN/m rather slowly. The value of y decreases from 72 to 22 mN/m in pure ethanol. In comparison, the value of y of surfactant solutions decreases to 30 mN/m with surfactant concentration around millimoles per liter (range of 1 to 10 g/L). Soaps have been used by humans for many centuries. In biology, one finds a whole range of amphiphile molecules (bile salts, fatty acids, cholesterol and other related molecules, phospholipids). 

Primary amides of unsaturated fatty acids have been characterized in humans and other mammals, and although their biological role is not fully understood, they may represent a group of important signalling molecules. Oleamide, the simple amide of oleic acid, has been shown to be a sleep-inducing lipid, and the amide of erucic acid, eru-camide, stimulates the growth of blood vessels. 

Mammals lack the enzymes to insert double bonds at carbon atoms beyond C-9 in the fatty acid chain. Thus they cannot synthesize linoleate and linole-nate, both of which have double bonds later in the chain than C-9 (linoleate has cis, cis A9, A12 double bonds, and linolenate has all-ris A9, A12, A15 double bonds). Hence, in mammals linoleate and linolenate are called essential fatty acids since they have to be supplied in the diet. These two unsaturated fatty acids are also the starting points for the synthesis of other unsaturated fatty acids, such as arachidonate. This C20 4 fatty acid is the precursor of several biologically important molecules, including the prostaglandins, prostacyclins, thromboxanes and leukotrienes (see Topic Kl). 

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