ISOELECTRIC POINTS 437 



At the isoelectric point the amount of ionized protein is 

 at a minumum, or, in other words, the protein is mainly 

 electrically neutral or uncharged. In this condition it will 

 not wander in an electric field or, as it is usually expressed, 

 exhibit cataphoresis ; moreover, in consequence of the ab- 

 sence of electric charge, the protein is most readily precipitated 

 at the isoelectric point. 



In this connection attention may be drawn to the hydrogen 

 ion concentration of the cell sap of various plants ; the values 

 obtained vary from 6-87 for Ficus carica to 3-19 for Riimex 

 acetosella, as may be seen by reference to a paper by Chibnall 

 and Grover,* but except in the case of unripe fruits or such 

 as normally contain much acid the value lies somewhere about 

 5-5-6-5. Chibnall f gives the following comparison between the 

 isoelectric points of some leaf cytoplasmic proteins and the 

 hydrogen ion concentrations of the contents of the leaf cells : — 



It would appear from these figures and from similar obser- 

 vations by Pearsall and Ewing, that the proteins of the plants 

 are bathed in a solution whose reaction is on the alkaline side 

 of their isoelectric points, except in the case of the vine ; 

 such proteins therefore function as anions. The conclusion 

 may be drawn that if the reaction of the cell sap is made more 

 acid, so as to approach the isoelectric point of the proteins, 

 these latter would tend to be precipitated with serious conse- 

 quences to the cells concerned. Hoagland and Davis J have 

 indeed shown in the case of Nitella that the reaction of the 

 sap remained constant and the cells suffered no injury so long 

 as the Pg of the external medium exceeded 4-4, but when 

 the acidity was increased beyond this value to 4-4 or less, 



* Chibnall and Grover : " Biochem. Journ.," 1926, 20, 108. 



t Chibnall : " J. Amer. Chem. Soc," 1926, 48, 728. 



X Hoagland and Davis : "J. Gen. Physiol.," 1923, 5, 629. 



