Guilliermond - Atkinson — 30 — Cytoplasm 



C H (acid function) 



Radical C H: 



N Hj (basic function) 



There is, therefore, an acid function COOH and a basic function 

 NHz and this amino acid is an electrolyte in both senses, being 

 able by dissociation to give an H+ ion as well as an OH— ion. In 

 the first case there will be : 



COO- H4- 



R C H: 



N H, 



and with the addition of one molecule of water, in the second : 



COOH 



R CH: 



N H3+ O H - 



When the medium is acid, the dissociation of the acid function is 

 diminished or blocked by virtue of the law of mass action, whereas 

 all the OH— ions leave the radical of the ampholyte and the basic 

 function is manifested. The ampholyte, then, is composed only of 

 R'H+ and behaves like a cation. In the electric field it will migrate 

 to the negative pole. When, on the contrary, the medium is basic, 

 the dissociation of the basic function diminishes or is blocked, 

 whereas that of the acid function increases. The ampholyte is now 

 constituted only of the anion R*OH-. It behaves like an anion 

 and migrates to the positive pole. 



Finally, and this is very important, for a certain intermediate 

 reaction between the two extremes, the dissociation of the acid 

 valence being equal to the dissociation of the basic valence, the 

 ampholyte is both an anion and a cation simultaneously and there- 

 fore behaves somewhat as though it did not have any electric 

 charge, and does not move in the electric field. This point of no 

 migration is called the isoelectric point. It is important to notice 

 that this isoelectric point does not generally correspond to chemical 

 neutrality but to a particular pB. (isoelectric pH indicated by the 

 symbol pHi). 



At their isoelectric point, the ampholytes show, as has been 

 demonstrated by Jacques Loeb, a series of special properties: a 



