ASER ROTHSTEIN 



77 



in order to fully characlerize the phenomenon. If further experiments do vali- 

 date the asymptotic relationship between K+ inflow and K+ concentration, 

 then it might be assumed that the data represent an interaction between K+ 

 and a cellular constituent which is a part of the ion transport system. It has 

 already been pointed out in a previous section that K+ forms a complex with 

 certain sites located on the outer surface of the cell. The question then arises 

 concerning the possible identity of these sites with those whose existence is 

 suggested by the kinetics of the transport. Fortunately, it is possible to prevent 

 interaction of K+ with the binding sites of the cell surface by adding appropriate 

 concentrations of bivalent cations. The latter, because of their much greater 

 affinity for the binding sites will completely displace K+. The uptake of K+ in 



10 20 30 40 50 60 



Fig. 7. Effect of Mg^^ on the 

 removal of K^ from the medium 

 by actively metaboHzing yeast 

 cells. 



Fig. 8. Changes in the pH of 

 the medium during fermentation 

 of glucose, as influenced by various 

 ions. 



the presence of Mg"*"^ proceeds at a rate which is only a little slower than that 

 in its absence, despite the fact that the concentration of Mn++ is sufficiently 

 high to displace virtually all of the K+ from the cell surface (fig. 7). Thus it 

 must be concluded that the K+ binding sites which have been discussed previ- 

 ously (page 72), play no essential role in K+-transport. Any interaction between 

 K+ and the transport system must occur in a location inaccessible to bivalent 

 cations, either deeper within the cytoplasm, or perhaps within the lipid phase 

 of the membrane. 



OTHER MONOVALENT CATIONS 



Other monovalent cations can be transported into the cell in exchange for 

 H+ by the same mechanism that transports K+. A simple method for demon- 



