240 HOWARD J. CURTIS 



the ions of an electrolyte to pass thi'ough. This can be created in the 

 laboratory by separating two different solutions by a membrane hav- 

 ing pores so small that one of the ions in one of the solutions cannot 

 pass through — for example, a solution containing an ionized protein 

 separated from a salt solution by a cellophane membrane. Even 

 after equilibrium has been established, a potential difference will exist 

 between the solutions. This has come to be known as a membrane 

 potential, and Nernst proposed the equation : 



^ = ^ln^^ (2) 



as a measure of this potential. It will be seen that the equation is 

 identical to equation (1) for the case in which the mobihty of one ion 

 is very much larger than the other, so Nernst's view of the membrane 

 potential would merely be a special kind of liquid junction potential. 

 On the basis of this equation the membrane potential would amount 

 to about 0.057 v. at room temperature for a 10 to 1 difference in con- 

 centration of the diffusible ion between the two sides of the membrane 

 at equilibrium. Whereas this formula has been verified in a general 

 way, it is certainly not completely correct. A number of efforts 

 have been made to place it on a more quantitative basis, none of 

 which has been completely successful. 



Certainly the best explanation of the potential difference that 

 exists between the inside and outside of living cells is to be found in 

 this theory. Due to measurement difficulties, which will be discussed 

 later, there has been very little actual verification of this. However, 

 such measurements as have been made tend to verify the general 

 concepts of the Nernst hypothesis, but leave many details unexplained. 

 According to this hypothesis, the living cell contains quite a high 

 concentration of large organic anions that cannot diffuse through 

 the cell membrane. Further, of the common cations found in bio- 

 logical systems, the cell membrane is presumed to be permeable 

 only to potassium. Under these conditions, equilibrium will be es- 

 tablished only when the concentration of K+ inside the cell is higher 

 than the concentration outside. Then the tendency of the K+ to dif- 

 fuse out and ecjualize the K+ concentration inside and out will be 

 balanced by the tendency of the K+ to diffuse in and neutralize the 

 excess anions inside. On this basis the membrane potential would be 

 given })y equation (2), where Ci and C2 represent the K+ concentra- 

 tions inside and outside the cell. And if one were to plot the log- 



