6 ELECTROLYTES IN BIOLOGICAL SYSTEMS 



amount of the medium adhering to the walls of the centrifuge tube, the inter- 

 cellular fluid volume contributes between lo and 20% of the water space 

 of the cell. 



Wash Losses From Resting Cells. If the radiosulfate immediately taken 

 into the cell is contained passively within a freely permeable membrane, then 

 the radioactivity should be readily washed out by immersion of the cells in a 

 saline solution. Such is the case as is shown in table 4. Cells immersed in saline 

 solution containing radiosulfate were harvested and reimmersed in saline 

 solution. The sulfur taken up by the cells from the first solution was then 

 found to be distributed between the cells and the second fluid, the washing 

 fluid gaining in radioactivity and the cells corresponding losing radioactivity. 

 A subsequent washing further reduced the remaining radioactivity of the 



Table 4. Wash losses of radiosulfate from cells into sodium chloride medium 



* 0.49 ml cells suspended in 2 ml saline solution containing carrier sulfate at the concen- 

 tration indicated for each tube. Wash volumes 5 ml in each case. 



t Original radioactivity per ml of saline solution before immersion of cells, 1,112 counts 

 per second. 



cells, a new equilibrium being established between the radioactivity of the 

 water space and that of the medium. It can be seen from the calculations of 

 the water space volume, both by the supernatant analysis method and from 

 the wash losses observed, that the sulfur concentration of the cells was always 

 equal to about 75% of the sulfur concentration of the medium. Little meta- 

 bohc binding occurred as shown by the ease of removing the sulfur by washing 

 and by the similarity of the results obtained over the wide range of sulfur 

 concentrations. If metabolic uptake occurred, its effects would be emphasized 

 at the concentration where the specific radioactivity was highest. It is evident 

 that metabolic uptake did not occur. 



Uptake During Growth. The uptake of radiosulfate during growth contrasts 

 sharply with that observed in resting cells. Radiosulfate uptake is directly 

 proportional to growth provided the water space sulfur has been removed by 

 washing the cells. This uptake as a function of growth is shown in figure 2. 

 A direct correlation between growth and the incorporation of sulfur into both 



