18 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



respectively. Each consists of glycerin in which the three atoms of 

 hydrogen in the hydroxyls are replaced by radicles of the fatty acid. 

 This is represented in the following formulae : — 



A /*q /J "Rn ni CLP "Pn f 



Palmitic acid Ci-Hji.COOH PalmitylCi5H3,.CO Pahnitin C3H,(OCi,H3i.CO)jj 

 Stearic acid C17H35.COOH Stearyl C17H35.CO Stearin CsH.XOCi^Hss.COJg 

 Oleic acid Ci,H33.COOH Oleyl Ci,H33.CO Olein C3H,(OCi,H33.CO)j 



Decomposition Products of the Fats. — The fats split up into the 

 substances out of which they are built up. 



Under the influence of superheated steam, mineral acids, and in 

 the body by means of certain ferments (for instance, the fat-splitting 

 ferment steapsin of the pancreatic juice), a fat combines with water 

 and splits into glycerin and the fatty acid. The following equation 

 represents what occurs in a fat, taking tripalmitin as an example : — 



C3H5(O.C,5H3,CO)3 + 3H20=C3H5(OH)3 + 3C,5H3iCO.OH 



[palmjtin — a fat] [glycerin] [palmitic acid — a fatty acid] 



In the process of saponification much the same sort of reaction 

 occurs, the final products being glycerin and a compound of the base 

 with the fatty acid, which is called a soap. Suppose, for instance, 

 that potassium hydrate is used ; we get — 



C3H5(O.C,5H3,CO)3 + 3KHO=C3Hg(OH)3 + 3C,5H3,CO.OK 



[palmitiii — a fat] [glycerin] [potassium palmitate — a soap] 



Emulsificatioii. — Another change that fats undergo in the body is 

 very difi"erent from saponification. It is a physical rather than a 

 chemical change ; the fat is broken up into very small globules, such 

 as is seen in the natural emulsion — milk. 



Lecithin (C42H84NPO9). — This is a very complex fat, which 

 yields on decomposition not only glycerin and a fatty {stearic) 

 acid, but phosphoric acid, and an alkaloid [N.(CH3)3C2H602] 

 called choline in addition. Lecithin is found to a great extent in 

 the nervous system,^ and to a small extent in bile. Together with 

 cholesterin, a crystallisable, monatomic alcohol (C27H45.HO), which 

 we shall consider more at length in connection with the bile, it is 

 found in small quantities in the protoplasm of all cells. 



• See further under Nervous Tissues, Lesson XXII. 



