96 ELECTROLYTES IN BIOLOGICAL SYSTEMS 



It occurs rapidly and is relatively non-specific. It involves no expenditure of 

 energy by the cell. The ions of the interior compartment which constitute the 

 bulk of the cell, participate only slowly in these exchanges. 



If a substrate is present, the K+ is pumped from the outer compartment into 

 the inner compartment by the carrier mechanism located in the inner mem- 

 brane which obtains its energy at least in part from the enzyme reactions of 

 the cell surface. The ion-pump mechanism shows a high degree of ion selectivity, 

 with K+ the most favored ion followed by Rb+ and then Na+, Li+ and Cs+. 

 As a consequence of the pumping of K+ from the outer zone to the inner zone, 

 there is a continued redistribution of ions between the medium and the outer 

 zone. K+ continues to move into the outer zone by diffusion in exchange for 

 H+, resulting in a very pronounced acidification of the medium. 



In the exchange of K+ and H+ it might be argued that K+ moves passively 

 to maintain electrical balance when H+ moves out of the cell. What might force 

 11+ out of the cell? Certainly the production of H+ in the cytoplasm does not 

 induce an outward diffusion into the medium, for the cytoplasm actually has 

 a lower H+ concentration by a factor of 100 to 1000, and it becomes even more 

 alkaline as the secretion of H+ proceeds. Nor could acid production in the outer 

 zone of the cell account for the ion movements. This could only produce a 

 high concentration of K+ in the outer zone in the exchange reaction and some 

 mechanism for pumping into the cell would still be required. Furthermore the 

 acidity of the outer zone would have to be intolerably high (pn 0.3) to account 

 for the observed movements of K+. Actually, on the basis of present evidence, 

 K+ can move against a greater concentration gradient than does the H+. Thus 

 it seems likely that the K+ is actively pumped and the H+ moves passively in 

 response to the electrical gradient which is established. 



The general buffer-pool of the outer zone may serve as the immediate source 

 of H+ which appears in the medium in exchange for K+. However, there are 

 only limited amounts of H+ available in the cellular buffers compared to the 

 substantial amounts that are excreted and the H+ must ultimately be derived 

 from the substrate. The interior of the cell is also involved because it becomes 

 more alkaline after the onset of fermentation. The buffer-pool of the cell is in 

 equilibrium with many kinds of metabolic reactions, which might serve as the 

 ultimate source of H"^ for exchange with K+. These include redox reactions 

 (as suggested by Conway (4)), production of organic acids, production of CO2 

 and phosphorylation reactions. The phosphate cycle continuously produces 

 and absorbs H+ due to differences in the dissociation constants of the various 

 phosphate compounds. For example, production of ATP from orthophosphate 

 or hydrolysis of ATP to orthophosphate involves considerable changes in H+ 

 concentrations depending on the pH. 



The concentrations of K+ and H+ in the outer compartment are consider- 

 ably different from those in the central compartment of the cell. The concen- 



