ASKR ROTHSTEIN 83 



is no evidence ihai any appreciable fraction of cellular K+ is chemically associ- 

 ated with polysaccharide. Probably the changes in polysaccharide and in K+ 

 content are both manifestations of a common metabolic function. 



There is no direct evidence that a major fraction of the potassium of yeast is 

 bound in an undissociated complex with any cellular constituents. True, there 

 are potassium binding substances which must be considered, particularly the 

 polyphosphates such as ATP and inorganic metaphosphates (69), and it has 

 been demonstrated that such substances on the cell surface can bind K+ to 

 the extent of i X io~^ m/1. of cells (54). However, the dissociation constants of 

 the K-polyphosphate compounds are relatively high, and the maximum 

 amounts of metaphosphate present in the yeast cell could bind only a fraction 

 of the cellular potassium. The bivalent cations of the cell would also tend to 

 occupy most of available sites because of their greater affinity for those binding 

 sites. Furthermore, the amount of potassium taken up and retained in the cell 

 is large when no phosphate is present (51) and little metaphosphate synthesis 

 occurs. Under some circumstances the amount of K+ taken up is greater than 

 the total amount of phosphate in the cell (51, 43). At most K+-binding could 

 account for only a minor fraction of K+-uptake and retention. On the other 

 hand, Conway (4) has shown that under certain conditions, the increased K+ 

 content associated with sugar uptake is balanced by an increase in organic 

 anions such as succinate or bicarbonate or both, depending on conditions. If 

 phosphate is present then a part of the K+ uptake is balanced by phosphate 

 uptake. Although phosphate is taken up at a much lower rate than is K+, the 

 K+ uptake stops after 10 to 20 minutes (51), whereas the phosphate uptake 

 may continue for a much longer period of time (19). Although the initial in- 

 crease in K+ may be largely balanced by metabolically produced anions, after 

 a period of time, the phosphate may play a more important role. 



The uptake of K"*" is closely related to acid-base balance because of the con- 

 comitant excretion of H+. More H+ ion is excreted than is formed. Therefore, 

 the pH of the cell rises slightly, from 5.8 to 6.2 (8). 



To summarize briefly, the yeast cell possesses at least three mechanisms for 

 actively transporting electrolytes against activity gradients. The first is con- 

 cerned with the uptake of K+ in exchange for H+ from the cell. Other mono- 

 valent cations can also be transported by this system, but there is a high degree 

 of specificity for K+. The second is concerned with the outward transport of 

 Xa+ in exchange for K+. The third is concerned with the uptake of inorganic 

 phosphate. Bivalent cations, if present, are apparently taken up by the latter 

 system as a i to i complex with phosphate. 



In considering the overall electrolyte balance, it is necessary to account not 

 only for the active transport of electrolytes from the environment to the cell, 

 but also for the leakage of electrolytes out of the cell and for the production of 

 electrolytes by metabolic reactions within the cell. The latter include H+, 



