PERMEABILITY AND THE PROTOPLASMIC MEMBRANE 285 



passage of ions of one electric sign while permitting ions of the 

 opposite sign to pass. As a result of this selective behavior, 

 electric stresses arise due to an ionic concentration gradient 

 across the membrane. We may, with Ostwald, ascribe the 

 electric forces at membrane surfaces to selective permeability, 

 or reverse matters and explain selective permeability in terms of 

 the electric forces. This is not arguing in a circle to the extent 

 that it may seem. Proteins possess a feeble residual charge 

 of their own even when quite free from electrolytes. Pauli 

 found blood serum to be weakly negative after seven weeks of 

 dialysis. The charge still remaining is due to ionization of the 

 protein (page 485). This weak initial charge of a protein 

 membrane is greatly augmented by the presence of salts. 



Two directly opposed hypotheses of the electric behavior of 

 membranes are equally plausible. They have been advanced by 

 L. Michaelis, W. R. Amberson, and others. The two hypotheses 

 are like the original one of Ostwald in that they ascribe the 

 entrance of ions of one sign and the holding back of those of the 

 opposite sign to a charge on the membrane. But which sign 

 must the membrane be to prevent this or that ion from passing? 

 If it is positive, it may repel cations and permit anions to pass or 

 attract and hold anions and permit cations to pass. It cannot, 

 therefore, in theory be said whether repulsion or attraction 

 (adsorption) determines the permeability of a charged membrane 

 for ions; only actual experimentation with membranes of known 

 sign will determine how a membrane functions electrically. 

 There is also the possibility of there being, as Ostwald said, a 

 potential gradient across the membrane, which would mean 

 that one side is negative and the other positive. 



Amberson, in observing reversal in the selective permeability 

 of membranes, was able to answer the question as to the way 

 in which a charged membrane may function. When the living 

 skin of the frog is removed from the body and bathed in Ringer's 

 solution, a difference of potential is produced across it, the inner 

 surface being electropositive. The frog skin probably consists 

 in large measure of protein; and as proteins are amphoteric, they, 

 and therefore the skin, will be positive when bathed in acid and 

 negative when bathed in alkali. The point at which this change 

 takes place is the isoelectric point and is expressed in terms of 

 acidity. The frog skin is negative when above (on the alkaline 



