CHAPTER LXXVII 



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 include fatty acids, neutral fats, cholesterols, cholesterol esters, and 

 phospholipids. 



The fit tt>i 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 -CH - 

 CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -COOH, or" CH3- 

 (CH 2 ) 14 -COOH; that is to say it is a higher member of the series to which 

 acetic acid (CH 3 -COOII) belongs, differing from the latter in having four- 

 teen extra methyl radicles, each joined to its neighbor by one bond or satu- 

 rated linking on either side. Another member of this series is stearic, in 

 which there are sixteen extra CH 2 groups (CH 3 (CH 2 ) lr ,-COOH). An un- 

 saturated fatty acid is oleic (CH,(CH 2 ) 7 — CH 29 = CH-(CH 2 ) 7 -COOH). 

 Its unsaturation is represented in the formula by the double bond or 

 unsaturated linking, which it will be seen occupies a position in the mid- 

 dle of the molecule, the other methyl radicles being linked together by 

 single bonds. 



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

 CH3(CH 2 ) 14 -COOH + KOH=CIT 3 (CH 2 ) ]4 -COOK + H 2 0, the reaction being 



(palmitic acid) (soap) 



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 form ethyl acetate, 

 CH3COO JH + OHi C 2 H 5 =CH ; COO-C 2 H, + H 2 0. When the alcohol thus 

 (acetic (ethyl (ethyl acetate) 



aeid) alcohol) 

 united with fatty aeid is glycerol (glycerine), in which there are three 



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