CHAPTER LXXIX 



FAT METABOLISM 



Before considering the physiology of fats, a few of the most essential 

 points regarding their chemistry may be of assistance. 



THE CHEMISTRY OF FATTY SUBSTANCES 



It is usual to classify all substances that are soluble in ether as lipoids. They in- 

 clude fatty acids, neutral fats, cholesterols, cholesterol esters, and phospholipins. 



The fatty acids belong to two main homologous series, which differ from each other 

 with regard to whether they are saturated or unsaturated. A saturated fatty acid 

 is typified by palmitic, whose formula is CH 3 -CH 2 -CH 2 -CH 2 -CH 2 -CH -CH o - CH^-CH. -CH o 

 CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -COOH, or CH-(CH 2 ) i4 -COOHj that is to' say 2 it is" a higher 

 member of the series to which acetic acid (CH -COOH) belongs, differing from the 

 latter in having fourteen extra methyl radicles, each joined to its neighbor by one 

 bond or saturated linking on either side. Another member of this series is stearic, in 

 which there are sixteen extra CH 2 groups (CH 3 (CH 2 ) i6 -COOH). An unsaturated 

 fatty acid is oleic (CH 3 (CH 2 ) 7 CH = CH-(aE 2 ) 7 -COOH). Its unsaturation is rep- 

 resented in the formula by the double bond or unsaturated linking, which it will be 

 seen occupies a position in the middle of the molecule, the other methyl radicles being 

 linked together by single bonds. 



The fatty acids readily combine with alkali to form soaps; thus, 



CH 3 (CH 2 ) i4 -COOH + KOH = CH 3 (CH 2 ) 14 -COOK + H 2 O, 

 (palmitic acid) (soap) 



the reaction being analogous to that by which acetic acid forms an acetate with 

 alkalies. In place of being combined with alkali, the COOH (carboxyl) group of fatty 

 acids may combine with alcohols to form substances called esters. Thus, acetic acid and 

 ethyl alcohol from ethyl acetate, 



CH coo ,'H + OH,' c H CH coo-c H +H o. 



3 \ _ | 2 5 3 252 



(acetic (ethyl (ethyl acetate) 



acid) alcohol) 



When the alcohol thus united with fatty acid is glycerol (glycerine), in which there 

 are three OH (hydroxyl) groups, the resulting ester called triglyccride is neutral fat. 

 Tripalmitin has the formula: 



CH,-OOC-C I5 H 31 



CH -OOC-C ]5 H 31 



CH 2 -OOOC 1B H 31 . 



By boiling neutral fats with alkali the fatty acid radicles are split off as soaps, 

 leaving the glycerol. This process is called saponification, and it may be effected in 



718 



