Chapter 4 



LIPIDES (FATS AND RELATED 

 SUBSTANCES) 



In every-day use the term fat has a fairly definite meaning. It sug- 

 gests such famihar substances as butter, lard, tallow, olive oil, cotton- 

 seed oil, and so on. However, if the distribution of fats in nature is 

 studied more closely, it soon becomes apparent that fats exist in less ob- 

 vious and less easily characterized combinations. Although the common 

 fats are essentially combinations of glycerol and fatty acids, many other 

 constituents are contained in fats and fat-like substances. Because of 

 this heterogeneity no very satisfactory classification of the fats has yet 

 been worked out. Perhaps the best one yet developed is that by Bloor 

 who bases his classification on three points: (1) solubility {e.g., insoluble 

 in water, soluble in ether, chloroform), (2) structure, i.e., esters of the 

 fatty acids, either actual or potential, (3) utilization by living organisms. 

 From Bloor's classification it is evident that fats or Hpides are essentially 

 ester combinations that yield various products on hydrolysis. The table 

 on page 73 gives examples of the different classes of lipides and the prod- 

 ucts formed by hydrolysis. This table gives a general view of the 

 lipides as a whole and should be used as a guide to which additions are 

 to be made as each class of lipides is studied more intensively. 



ESTERS 



Definition 



Since the lipides consist, for the most part, of esters, it is important 

 for the student to understand clearly just what is meant by the term 

 ester. An ester is a substance formed by the chemical reaction of an 

 alcohol with an acid, whereby a molecule of water is eliminated. The 

 formation of a simple ester is represented by the following equation: 



O O 



II 11 



CHsOH + HOCCH, ^=^ C2H5OCCH3 + H,0 



Ethyl alcohol Acetic acid Ethyl acetate Water 



71 



