ASER ROTIISTEIN 79 



sources of H+ within the cell, so that the substrate must serve as an ultimate 

 source. There are many potential sources of H+ in the metabolic scheme such 

 as organic acid production, redox reactions, and phosphorylation reactions. 

 Despite the large turnover of H+ associated with these sources, the pH of the 

 cell remains relatively constant, partly because of the high concentrations of 

 buffers in the cytoplasm and partly because of acid-base regulating systems 

 which prevent imbalances. Measurements indicate that the average pii of the 

 cytoplasmic contents of a resting cell is about 5.8. During the active metabolism 

 in the presence of K+ the cell becomes more alkaline, rising to pH 6.2. The H+- 

 production from metabolism is more than balanced by the excretion of H+ in 

 exchange for K+ (8). Furthermore, the cytoplasmic pn is independent of extra- 

 cellular pH over a wide range (49). 



The mechanism of H+ excretion cannot be considered without taking into 

 account the mechanism for the uptake of K+. Under usual conditions both ions 

 move against large concentration gradients. However, it is not necessary that 

 both be actively transported. For example, H+ may move outward on the 

 electrical gradient established by the active inward transport of K+. Or the 

 reverse situation may exist. It is also possible that both ions are actively 

 transported. The data necessary to unequivocally differentiate the alternatives 

 listed above is not yet available. 



ANIONS AND BIVALENT CATIONS 



When yeast is suspended in glucose plus KCl, there is a stoichiometric rela- 

 tionship between K+ uptake and H+ secretion. It is apparent therefore that CI 

 does not participate in the ion transfers. This has been confirmed by direct 

 measurements of Cl~ (17, 4). The metabolizing cell is also impermeable to such 

 anions as succinate and citrate although undissociated succinic acid can be 

 excreted. However, if inorganic orthophosphate is used as an anion, a more 

 complicated situation exists. Although the resting cell neither takes up, loses, 

 nor exchanges phosphate to an appreciable extent, the actively metabolizing 

 cell takes up appreciable amounts (29). Furthermore, the uptake is remarkably 

 enhanced if potassium is present (62, 18). The phosphate uptake is not simply 

 due to an increased permeability associated with metabolism, for it proceeds 

 against apparent concentration gradients of phosphate as great as 100 to i. 

 Only the monovalent anion, H2P04~ is transported. The kinetics of phosphate 

 uptake indicate that the process involves a combination of phosphate with 

 some constituent of the cell (19). Thus in figure 9, the rate of phosphate uptake 

 is plotted as a function of phosphate concentration. The relationship is asymp- 

 totic and can be fitted by the Michaelis-]\Ienten equation. Such curves usually 

 indicate that the rate is dependent on the combination of the substrate with a 

 cellular constituent present in limited quantity. 



Despite the rapid inflow of phosphate associated with metabolic activity, 



