CHAPTER XXIV 



FATS 



Fats play their important part in the hfe of the cell and of the 

 organism as a whole through protection (against cold, injury, 

 and drying out) and as an important source of reserve energy. 

 They have a further significant role in cellular activities in that 

 they, in great measure, determine selective permeability. Fats 

 constitute, in animals, the principal and, in plants, an important 

 fund of reserve food, but their value as dietary substances is 

 even greater; they may function as, or be the source of hormones 

 and vitamins. It is in this respect that fats and fatlike sub- 

 stances have received particular attention of late. 



The word "fat" will be used here to include not only the fats 

 proper — the fatty-acid tri-esters of glycerol — but also other 

 fatlike substances which are of so much interest to the biologist. 

 Certain of these latter substances are grouped together as 

 lipoids. The term is an old one and in common use, but it has 

 met with considerable opposition because it brings together a 

 somewhat diverse group of substances. Substitutes for it, such 

 as lipin and lipide, are no more satisfactory, for they too have 

 been variously used; thus, Bloor recommends "lipide" as a 

 general group name to include the true fats and the lipoids. 

 "Oil" is another term the meaning of which is not definite. 

 It is best distinguished from fat by regarding the simple lipides, 

 i.e., the esters of the fatty acids with glycerol, as oils when liquid 

 and fats when solid. While such a terminology might call for 

 another word as the all-inclusive group term, for which purpose 

 Bloor has suggested lipide, yet until the nomenclature is definitely 

 decided upon, fat will serve better than any other word to convey 

 a conception of the qualities possessed by those substances 

 listed below. 



Fats are of three kinds, the simple ones, or true fats, which 

 are esters of the fatty acids with either glycerol (the fats proper) 

 or alcohol (the waxes) ; the compound ones, which are chiefly the 



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