78 ELECTROLYTES IN BIOLOGICAL SYSTEMS 



strating the relative rates of ion uptake is shown in ligure 8 (48). Cells are 

 suspended in an unbuffered medium at pH 4.5. Glucose alone or glucose plus a 

 given salt (chloride) is added and the pn is measured continuously. With glucose 

 alone, the pn drops to 3.5 due to organic acid secretion and remains there. If 

 K+ is added the pH drops more rapidly to a much lower value due to the out- 

 ward exchange of H+. With other monovalent cations, the amount of H+ 

 eventually excreted is about the same as with K+, but the rate of excretion is 

 considerably lower than for K+. Rb+ is taken up about one-half as fast as K+; 

 Na+, Cs+, and Li+ are taken up at a lower rate. Large organic cations such as 

 (C2H5)3N+ and bivalent cations such as Mg++ and Ca++ are not carried into the 

 cell by the H+-exchange mechanism. If Na+ and K+ are present at the same 

 time, they compete for the transport mechanism, but K+ is favored by a factor 

 of about 20/1 (5). 



Although the inward ion transport system possesses a high degree of speci- 

 ficity for K+ as compared to Na+, there is another transport system which is 

 relatively specific for Na^" (5). Thus if cells are made rich in Na+ by allowing 

 them to metabolize glucose in the presence of high Na+ concentrations, but no 

 K+, and if these cells are resuspended in a medium containing K+, cellular 

 Na+ is expelled in exchange for K+. Conway (5) believes that the Na-excreting 

 system is independent of the K-uptake system not only on the basis of ion 

 specificity, but also on the basis of studies with inhibitors, some of which in- 

 fluence one system and not the other. It is also interesting to note that certain 

 steroids influence the excretion of Na+ by yeast but not the uptake of K+ 

 (10). 



It has been pointed out that H+ ion is liberated by the yeast cell in exchange 

 for certain cations. However, H+ can also be excreted by the cell in the absence 

 of cation uptake. For example, in the control experiment of figure 8, with no 

 cation added, the pH of the medium drops from 4.5 to 3.7. It has been pointed 

 out previously that this is due to the excretion of undissociated organic acid, 

 largely succinic acid. If K+ is present, the excretion of succinic acid is largely 

 suppressed in favor of K+-H+ exchange. However, the uptake of potassium 

 does not proceed indefinitely, but reaches a maximal value of about 0.08 m/1. 

 of cells in about 20 minutes. Thereafter if the pH of the medium is above pn 3.5, 

 succinic acid is excreted even in the presence of K+. In fresh yeast some of the 

 secreted succinate may be present in the cells at the start of the experiment 

 with the remainder being derived directly from the substrate (6). In well 

 starved cells, the succinate level is reduced to a low level and succinic acid 

 secretion is also reduced. However, succinate depleted cells can still exchange K+ 

 and H+ at a maximum rate. For this reason, among others, Conway (4) rejects 

 the concept that organic acids produced by metabolism serve as a direct source 

 of H+ for exchange with K+. 



The amount of H+ that can be excreted is large relative to the potential 



