LIPIDES (fats and RELATED SUBSTANCES) 97 



their components, which inchide fatty acids or aldeliydes, phosphoric 

 acid, a nitrogenous base, and a polyhydroxy alcohoh PhosphoHpidcs 

 may be divided into three subgroups according to the alcoholic com- 

 ponent, as follows: 



glycerol: lecithin, ccphalin, plasmalogen 



inositol: lipositol, mono- and diphosphoinositides 



sphingosine : sphingomyelin 



The lecithins and cephalines arc the best known and most important 

 members of the entire group. 



Phospholipides are believed to occur in every cell and are particularly 

 abundant in some of the most important and active tissues of the body: 

 brain, liver, and mammary gland. They appear to be an essential part 

 of the actual cell structure, and not merely stored-up food as are the 

 true fats in adipose tissue. This essential character is indicated by the 

 fact that the amount of phospholipides in the tissues is not materially 

 reduced during extreme starvation. For this reason these substances are 

 often called "essential lipides," or the "nonvariable component" of tissue 

 lipides, in contrast to the true fat, or "variable component." 



Lecithins 



The structure of a lecithin may be illustrated by the formula: 



O 



II 

 H2C — — C — CijHsi 



o 



11 



HC — O — C — C17H33 



o 



II 



H2C-O— P— OCH^CH^NCCH,), 

 I + 



o- 



This formula shows that the substance is a glyceride in which one fatty 

 acid radical has been replaced by a group consisting of phosphoric acid 

 combined as an ester with choline (p. 162). Since the phosphate residue 

 is strongly acidic and the choline nitrogen strongly basic, the two neu- 

 tralize, each other and form an inner salt, or zwitterion, which is indicated 

 in the formula by the + and — signs. The fatty acids in lecithins tend 

 to be rather highly unsaturated, although some saturated acids are usually 

 present. A hydrolecithin, in which bot-li fatty acids (palmitic) are satu- 

 rated, has also been found in brain and lung tissue. It is much less 

 soluble in the usual fat solvents than unsaturated lecithins. It is theo- 

 retically possible to have lecithins in which the phosphoric acid-choline 



