350 PROTOPLASM 



membrane. The former may be permanent, but the latter 

 must be constantly changing in order to maintain itself and is 

 therefore a Ukely condition in living systems. 



The migration of ions, whether freely in a solution, selectively 

 through a membrane, or under the influence of an electric field, 

 involves the question, How far can an ion get away from its 

 mate? It has already been stated that one ionic species cannot 

 separate far from the other without setting up a very great 

 potential difference. In a voltaic cell, or in electrolysis wherever 

 it occurs, positive ions move in one direction, and negative ions 

 in the opposite direction. They probably do not travel freely, 

 i.e., wholly alone, but pass from one ion to another. They 

 thus leave their original mates and pass on to others. In saying, 

 therefore, that a membrane is selectively permeable to cations— 

 that the apple skin permits potassium to pass through more 

 readily than chlorine — it is not intended to imply that there is a 

 true and complete separation of the ions without other ions 

 taking the place of the discarded mates. There is an interchange 

 of ions between the two sides of the membrane. Electric 

 neutrality (equilibrium) must be maintained in static systems 

 and striven for in dynamic (living) systems. This difficulty 

 may be avoided by assuming that there is no abrupt change in 

 the potential at the surface of the membrane but a potential 

 gradient from one side to the other. The membrane does not 

 sharply separate two ionic species and therefore two surfaces of 

 different potential; there is, instead, a gradual transition between 

 the potentials of the two sides. 



The role of membrane potentials in vital phenomena is probably 

 a very great one, because there are many membranes in hving 

 systems, and each is the seat of an electric potential. All hving 

 membranes, whether the covering of tissues, of cells as a whole, 

 or of cell parts, are differentially permeable and are bathed in 

 electrolytes. This is sufficient to indicate that hving cells 

 are generators of electromotive forces. Theoretically, this must 

 be true, but there is experimental evidence as well. The apple 

 skin is more freely permeable to cations than to anions. Potas- 

 sium passes through more readily than chlorine. As a result, 

 the tissue within the apple is left with a negative charge, and 

 the outer solution becomes positive. That a potential is actually 

 established can be proved by measurement, as has been done by 



