8o 



ELECTROLYTES IN BIOLOGICAL SYSTEMS 



there is little exchange of phosphate between the cell and the medium as 

 measured by isotope technique (19). For example in figure 10, the disappearance 

 of chemical phosphate and P^^ from the medium follow parallel curves. There 

 is little change in the specific activity of the phosphate remaining in the medium. 



150 - 



PO4 CONCENTRATION IN M/L X 10 



1 I 1 



2 4 6 8 10 



Fig. 9. The effect of ])hosphate concentration on the rate of phosphate uptake by yeast. 



PO4 UPTAKE 



Fig. 10. The uptake of phos- 

 phate by cells actively metabo- 

 Hzing glucose. 



-4003 



Thus there has been no dilution of P^'- with natural phosphate leaking out of 

 the cell. In other words, phosphate ions are taken up, but none are given out. 

 This is in sharp contrast to the situation with respect to potassium, in which 

 there is, in the metabolizing cell, both an exchange of K+ between the cell and 

 the medium and a net movement of the ion into the cell. 



The maximal rate of uptake of phosphate is only about 30 per cent of the rate 

 of potassium when both are present at the same time. Thus even in a phosphate 



