68 



ELECTROLYTES IN BIOLOGICAL SYSTEMS 



BINDING OF IONS BY THE SURFACE OF THE CELL 



Although extracellular ions equilibrate only at a slow rate, if at all, with the 

 total water of the resting yeast cell, they do equilibrate rapidly with a surface 

 layer of the cell. Conway (9) found that a variety of substances including both 

 electrolytes and non-electrolytes could distribute readily in an outer zone of 

 the cell equal in volume to about 10% of the total cell volume. An additional 

 factor is the rapid binding of cations by fixed anionic groups of the cell surface. 

 The ion-binding properties of the cell surface of yeast were first studied with 

 uranyl ion (52, 55). Small quantities of U02"^ are almost entirely taken up 

 from the medium by the cells. However, if the amount of U02"^ is increased 

 above a certain value the cell becomes saturated and will take up little more 

 of the cation. For example, in figure i over 90% of the U02"^ is taken up at 



Fig. I. The relationship be- 

 tween the final concentration of 

 U in the medium and the U-up- 

 take i^er cell after one hour. 



FINAL U- CONCENTRATION IN MEDIUM IN MOLS / LITER X lo' 



concentrations of 1102"'^ below i X io~^ m/1. but there is little additional up- 

 take at concentrations above 2 X io~^ m/1. The maximum uptake amounts to 

 about I X io~^ m/1. of cells. If it is assumed that each U02+^ is bound by one 

 cell site, then the concentration of cell sites is also i X io~^ m/1. of cells. 



The binding of uranyl ion by the cell is a completely reversible process that 

 ran be represented by the simple equation, 



U + Y ;z± UY 



(i) 



where U represents 1102"'^ 

 law form, the equation is, 



Y a yeast site, and UY the complex. In the mass 



K = (U) (Y)/(UY) 



(2) 



The data of figure i fit equation 2 very well. The calculated dissociation con- 

 stants for each of 10 points in the rising part of the curve (the points on the 

 plateau cannot be used for this calculation) given in table 2, range from 3.0 

 to 5.3 X lo"''. In making the calculation it should be noted that the activity 

 coefficient of the UOo"^ is taken as i which is essentially correct for the very 

 dilute solutions used. The activity coefficients of the two solid phases Y and 



