70 



ELECTROLYTES IN BIOLOGICAL SYSTEMS 



U02++-binding sites of the cell were chemically related to the polyphosphates. 

 In addition to the polyphosphates, a second species of UO^"*"^ binding site was 

 also present on the cell surface of yeast, which possessed a considerably lower 

 affinity for the cation. 



A number of other bivalent cations were found to compete with U02"'"'" for 

 the cell-surface sites (58). However, relatively high concentrations of the 

 competing cations were required to displace the UO-"'^. The binding of other 

 cations was studied in more detail using isotope technique with Ca** and es- 



80 - 



60 



40 



20- 



TIME IN MINUTES 



20 40 60 80 100 120 140 160 



Fig. 2. The time-course of uptake of Ca"*^ by yeast cells. 



180 



60 uj Fjg. 3. The uptake of Mn^'* by 

 K yeast cells as a function of Mn++ 

 40 = concentration. 



2 4 6 8 10 12 14 



EQUILIBRIUM CONCENTRATION OF Mn** IN M/L x 10^ 



pecially with Mn^"* (54). As with 1702+^, the binding of Ca++ and Mn++ was 

 reversible and reached equilibrium rapidly. Within 2 minutes, the earliest 

 observation, the reaction was essentially complete (fig. 2). Mn++ is not bound 

 by the cell in such a stable configuration as UOo"*^. For this reason, the curve 

 representing Mn++ uptake as a function of Mn++ concentration does not rise 

 so sharply to an asymptote (fig. 3). As with 1102"^"'', the binding of Mn++ can 

 be expressed by a simple relationship: 



^ _ (M)(Y) 



0) 



