2 ELECTROLYTES IN BIOLOGICAL SYSTEMS 



may not enter into the metabolic activities of the cell. However, we believe 

 that such puzzling effects should not be attributed to 'impermeability' of the 

 cell membrane unless it is shown that the molecule does not penetrate (23). 

 Radioactive substances ranging from inorganic ions to peptides have been 

 used in the investigations to be described, and with Escherichia coli and Toru- 

 lopsis utilis the same water space was found for each of the different com- 

 pounds tested. Other evidence demonstrating the permeability of E. coli, 

 T. ulilis, and Neurospora crassa supports the conclusion that in these organisms 

 passive diffusion is entirely adequate as a mode of transport. 



Table i. Immediate radiosulfate 

 uptake from complete medium* 



* Synthetic medium containing glucose 

 and aerated at 37°C. 



t The same quantity of radioactivity 

 was added to each flask. 



Table 2. Immediate radiosulfate 



UPTAKE from saline SOLUTION 



* 0.026 mg sulfur/ml of saHne solution 

 (0.85%) was used in each experiment. 



RESULTS 



Measurement of Water Space. The distinction between metabolized and 

 water space materials appearing within the confines of the cell membrane can 

 be clearly shown through studies of the uptake and incorporation of sulfur 

 by E. coli. In this species growth is required for the formation of nondiffusible 

 sulfur compounds (10). Without growth, sulfur uptake can only occur by 

 diffusion into the water space of the cell. This uptake occurs immediately 

 when resting cells are immersed in a complete medium containing radiosulfate, 

 and its measurement serves as a quantitative determination of the water 

 space volume. 



The immediate uptake of radiosulfate is shown in table i. In these experi- 

 ments I ml of resting cells was washed in 0.85% saline solution and then 

 suspended in synthetic medium- containing a known quantity of S^^-labeled 

 Na2S04. The cells were immediately harvested by centrifugation at 14,000 g. 

 The supernatant solution was decanted from the packed cells and the pellet 

 and tube were rinsed with 20 ml of saline solution. This rinse was carried out 

 by pouring the solution into the centrifuge tube containing the packed pellet 



2 6g NaoHP04, 2g NH4CI, 3g KH2PO4, 3g NaCl, 10 mg Mg (Mg CL.), 26 mg S (Na2S04) 

 and 900 ml water. 



