196 INTERMEDIATE METABOLISM 



triglycerides, and since the properties of each triglyceride 

 depend upon its fatty acid constituent, the properties of 

 a mixed fat depend upon the proportion in which saturated 

 and unsaturated fatty acids are present. So by estimating 

 the melting-point and the iodine number of a mixed fat 

 we have a measure of the degree of saturation of the fatty 

 acids composing it. 



Complex Lipoids 



Phosphatides {Phospholijnnes). — These may be regarded 

 as fats in which one fatty acid molecule is replaced by 

 phosphoric acid, by means of which it is hnked to the 

 base Choline — 



C2H4OHX /CH3 

 >N^CH3 



OW ^CHg 



The most important member of this series is Lecithin — ■ 

 CH2OOC15H31 



I 



I 



CHoO. .0 



OH^ ^OC2H4. /CH3 



>NfCH3 



Lecithin and allied substances form a constituent of all 

 living cells. 



Cerehrosides {Galactolipines) are compounds of fatty acids 

 with nitrogen and galactose. They occur largely in nervous 

 tissue. 



Lipoids may therefore be said to exist in the body in two 

 forms, as simple triglycerides and as more complex bodies. 



