Molecular Structure in Protoplasm 



59 



ered acetal-phospholipids (78) . Lecithin may be thought of as a 

 modified fat in which one of the three fatty acid radicals is replaced 

 by phosphoric acid plus a nitrogenous group, choline. Figure 6 



shows a model of lecithin with its 

 characteristic polar and nonpolar 

 regions. Approximately half of the 

 phospholipids found in typical animal 

 cells are lecithins. The other half is 

 composed largely of cephalin, which 

 differs from lecithin mainly in the fact 

 that the choline portion is replaced by 

 colamine, or amino-ethyl alcohol. 



Concerning the ionization of the 

 phospholipids it has been pointed out 

 (79) that lecithin and cephalin ionize 

 as shown in the diagram in Figure 7. 

 The isoelectric point of the former is 

 about pH 6.7 (80) . It seems probable 

 that in the case of lecithin both groups, 

 phosphoric acid and choline, are ion- 

 ized at the pH of cytoplasm (79, 81, 82, 

 83) . Cephalin, on the other hand, has a 

 weaker basic group (84) and may be 

 only partially ionized in cytoplasm. This makes it possible for certain 

 of the cephalin molecules to exist with a negative charge. 



For many years organic chemists (71) struggled with a large 

 group of fatty compounds which apparently bore some structural 



FiG. 6. Model photograph of 

 a lecithin molecule. Length 

 parallel to hydrocarbon chains 

 is equal to approximately 

 25A. 



I 



R, 



B 



o=c c=o 



I I 



O O H O H H 



III 'I II 



(chX H— C — C-C — 0-P-O-C — c-nh, 



III I II 



H H H 



,0 



H 



Fig. 7. Diagrams to show ionization of lecithin and cephalin. A. Zwitterion 

 form of lecithin. B. Zwitterion form of Cephalin. 



relation to one another even though they were widely different in 

 their physiological effects (sex hormones, etc.). It is due to the 

 efforts of the X-ray analyst, however, that this work crystallized into 



