Lipid 



es 



4 



When living tissues are treated with ether, alcohol, petroleum 

 ether, or chloroform, a number of substances are extracted, among 

 which is a group of compounds called lipides. Although the various 

 members of this group are quite often chemically unrelated, they do 

 have the following properties in common: (1) they are all appreciably 

 soluble in one or more of the solvents mentioned above, whereas most 

 of them are relatively insoluble in water; (2) they all exist as actual 

 or potential esters of fatty acids; (3) they can be utilized by a living 

 organism. 



When lipides are classified, they are usually divided into three main 

 groups, namely, simple lipides, compound lipides, and hydrolytic 

 products of lipides. Simple lipides are defined as organic esters which, 

 upon hydrolysis, yield only aliphatic alcohols and aliphatic mono- 

 carboxylic acids. Included in this category are the fats and waxes. 

 Upon hydrolysis, compound lipides yield aliphatic alcohols, aliphatic 

 monocarboxylic acids, and another product, such as a carbohydrate, 

 phosphoric acid, or a nitrogenous base. Examples of this group are 

 the phospholipides and the glycolipides. The hydrolytic products of 

 lipides include the fatty acids, various alcohols, such as glycerol and 

 the sterols, and a number of nitrogenous compounds such as choline 

 or sphingosine. 



FATTY ACIDS 



Occurrence 



Fatty acids are integral constituents of every simple and compound 

 lipide. In the majority of instances they are found in nature as esters 



74 



