82 



ELECTROLYTES IN BIOLOGICAL SYSTEMS 



In contrast to the non-specificity of the energy source for potassium uptake, 

 phosphate uptake, and the associated uptake of bivalent cations specifically 

 requires the metabolism of sugars, either aerobic or anaerobic, and is not in- 

 duced by the respiration of alcohol, acetate or pyruvate (i8). As with potas- 

 sium, however, the coupling of ion transport to the energy source is not obliga- 

 tory nor is it stoichiometric. Less than 14 molecules of phosphate or of bivalent 

 cations are taken up for each 100 molecules of sugar. 



As in the case of K"^ transport, a number of inhibitors such as DNP and 

 sodium azide can block the uptake of phosphate and of bivalent cations without 

 reducing the rate of substrate utilization or gas exchange (29, 43). These same 

 inhibitors can uncouple phosphorylation from oxidation in cell free enzyme 

 systems (^s)- 



Table 5. Back exchange of mn++ bound to the surface compared to that of mn++ 



taken up with phosphate 



Concentrations were as follows: K+, 2 X 10 ^ m/L; phosphate, 2 X 10 ' m/I.; and glucose, 



0.1 m/1. The pH was 4.5 and the j-east concentration 100 mg/ml. 



OVER.A.LL ELECTROLYTE REGUL.A.TION 



When Pulver and \'erzar (42) first observed K+ uptake, they used small 

 amounts of sugar. After the sugar was all respired, most of the K+ returned to 

 the medium at a slow rate. However, if larger amounts of sugar are used then 

 some of the K+ is retained by the cell (51). 



All of the factors involved in K-retention are not altogether clear, but there 

 is a significant correlation between the assimilation of sugar (conversion to 

 polysaccharide) and the amount of retained K+. Furthermore, during the follow- 

 ing period of starvation, as the carbohydrate stores are diminished, there is a 

 concomitant loss of K+ from the cells (51). The relationship of K+ content of 

 yeast to the dissimilation of carbohydrate has been studied in some detail by 

 Scott el al. (63). They found that inhibitors such as NaF and iodoacetate could 

 increase the rate of K+ loss. With NaF, the K+ loss is offset by a gain of Na+. 

 In any case, although carbohydrate depletion and K+ loss follow a somewhat 

 parallel course, the relationship is far from stoichiometric. Furthermore, there 



