ASER ROTHSTEIN 



8l 



coiuaining medium only abt)ul 30 per ceiU of the rale of uptake of polassium 

 can be associated directly with the uptake of phosphate. The rest is associated 

 with an exchange for hydrogen ion. 



On the other hand, the uptake of bivalent cations by yeast seems to be almost 

 entirely associated with the uptake of phosphate. Schmidt et al. (62) found that 

 phosphate uptake was enhanced not only by potassium, but by magensium 

 as well. Considerable quantities of magnesium were taken up in these experi- 

 ments. The uptake of bivalent ions was investigated using Ca'*^ and Mn^^ (53)- 

 As already pointed out, the surface of the cell will very rapidly bind these ions 

 in a reversible complex to a maximal extent of i mM/1. of cells. This amount is 

 equal to about 3% of the normal bivalent cation content of the cell. In the 

 {presence of glucose plus phosphate additional quantities of bivalent cations are 



Table 4. Binding and uptake of mn*^ as influenced by various factors 



Concentrations were as follows: Mn, 7.5 X 10 '' m/1. K, 2 X 10 ^ m/1.; phosphate, 2 X 

 io~' m/1.; glucose, o.i m/1. The pH was 4.5 and the yeast concentration, 100 mg/ml. 



absorbed. If K+ is also added, the uptake is markedly increased. In fact, in the 

 e.xperiment of table 4 all of the measureable Mn++ was taken up. In other experi- 

 ments the cellular content of bivalent cations was doubled in an hour. The 

 bivalent cation transport system is relatively non-specific at least with respect 

 to Mn"*^, Mg"*"^, and Ca++. It proceeds at concentrations of Mn+~^ as low as 

 I X 10-^ m/1. 



The bivalent cations are probably carried into the cell by the phosphate- 

 transport system as a i to i complex with H2P04~". The following evidence can 

 be cited: a) no uptake is observed unless a simultaneous uptake of phosphate 

 occurs; b) at low ]\In++ concentrations, relative to phosphate, there is less 

 Mn++ taken up than phosphate; and c) as the ]\In++ concentration is increased, 

 the ratio of Mn++ to H2P04~ approaches but never exceeds i.o (53). Bivalent 

 cations once carried into the cell in association with phosphate are no longer 

 exchangeable with the bivalent cations of the medium. This is in contrast to 

 bivalent ions which are simply bound on the cell surface (table 5). The latter 

 e.xchange very rapidly and completely with ions of the medium. 



