THE BIO-CHEMISTRY OF ANIMALS AND PLANTS 113 



longing to the three first groups of Thudichum's classification 

 seems therefore justified in this connection. These groups 

 comprise : 



1. The Phosphatides (containing P and N, such as lecithin, 



kephalin, etc.). 



2. The Cerebrosides (P free, but containing N, such as 



phrenosin, kerasin). 



3. The P and N free substances (cholesterin, inosite, etc.). 

 1. The Phosphatides are divided into sub-groups according 



to their N : P ratio. The best known substances belong to the 

 two groups of {a) mono-amino-phosphatides (N : P=i : 1) and 

 {b) di-amino-phosphatides (N : P = 2 : 1). 



Lecithin (or rather the lecithins) belongs to the first group 

 of mono-amino-phosphatides, and although we are able to form 

 what seems to be a fairly accurate picture of its constitution, 

 it is doubtful if any worker has ever isolated a pure lecithin. It 

 represents a waxy, hygroscopic, white or yellowish mass which 

 easily oxidises on exposure to air. On hydrolysis by means 

 of baryta, it gives rise to glycero-phosphoric acid, choline, and 

 fatty acids, and from these facts the following formula has been 

 derived : 



CH 2 PH0 3 -C. 2 H 4v 



I " \ 



*CH . O - Fatty acid radicle (CH 3 ) 3 - N 



CH 2 - Fatty acid radicle O H 



The fatty acid radicle may be variable. Stearic, palmitic, 

 oleic and linoleic acids have been obtained, and our knowledge 

 about the distribution of these acids in different lecithins is very 

 vague. 



Ulpiani has recently demonstrated the optical activity (dextro- 

 rotation) of lecithin from eggs. As the natural glycero-phos- 

 phoric acid, isolated from lecithin, was also found to be optically 

 active by Willstatter, it must possess an asymmetric C-atom 

 as indicated by the asterisk in the formula just given. Meyer 

 was able to convert the optically active lecithin into the racemic 

 inactive modification, and succeeded in obtaining from it the 

 laevo-rotatory form by the action of lipase. 



From a biological point of view it is interesting that phos- 

 phatides combine easily with many organic substances such as 

 carbohydrates, proteins, glucosides, alkaloids, and some of these 

 combinations occur preformed in animal and plant organs. The 



8 



