88 ELECTROLYTES IN BIOLOGICAL SYSTEMS 



metabolism are located in a more peripheral zone where they are accessible to 

 the H+ of the medium. 



The effects of K+ on metabolism can also be attributed to cell-surface reac- 

 tions. In the lirst place, it has already been pointed out that K+ competes with 

 H+, and that the effects of the latter are on the cell surface. In the second place, 

 the intracellular K"^ concentration may be readily decreased to 0.07 by starva- 

 tion or raised to 0.20 by fermenting in a high K"'"-medium, with little change in 

 the rate of fermentation. Yet the addition of much lower concentrations of K+ 

 to the medium markedly stimulates fermentation, even though no K+ is taken 

 up by the cell during the course of the experiment (48). Thus, the rate of 

 metabolism is influenced by extracellular rather than intracellular concentra- 

 tions of K+. 



Table q. Effect of bivalent ions on rate of fermentation of glucose at 



DIFFERENT PH's 



Rates are expressed as n\. of C02/mg of yeast (wet weight) per hour. Ion concentrations 

 were .01 m/1. Buffer triethylamine (TEA), succinate tartrate at pH 3.5 and 6.0, and trishy- 

 droxyaminomethane (THAM) at pH 8.5. Glucose concentration was o.i m/1. 



* Triethylamine chloride. 



Bivalent cations also influence the rate of metabolism by combining with the 

 fixed anionic groups of the cell surface. For example, 1702"'"^ in low concentra- 

 tions completely blocks the fermentation of glucose. In somewhat higher con- 

 centrations, UO2++ will block the respiration of glucose, but not the respiration 

 of alcohol, pyruvate, lactate or endogenous stores (59). The inhibition of fer- 

 mentation is associated with the formation of a uranyl complex with the poly- 

 phosphate groups of the cell surface, whereas the inhibition of respiration is 

 associated with polyphosphate groups only to the extent of 60%, and with 

 carboxyl groups in the case of the remaining 40%. The UOo"*^ inhibition can be 

 competitively reversed by Ca++, Mg++ or ]\In++, indicating that the surface 

 groups when combined with the latter ions, are functional in sugar uptake. 

 In fact, the normal fermentation can be stimulated by each of these ions (table 

 9). As with K+, the effect is markedly dependent on the extracellular pH. In the 

 pH range 2.0 to 6.0 Mg++, Mn"*"'" and Ca++ each will stimulate 15 to 20%. At 

 pH 7.0, they increase the fermentation to a much greater extent. But at pH 

 8.5, Mg"^ and Mn++ have no effect, whereas Ca++ inhibits about 30% (49). 



