8 



ELECTROLYTES IN BIOLOGICAL SYSTEMS 



sulfur occurred in these experiments also indicates that the sulfur removed 

 from resting cells (tables i, 2) by washing did not contain large quantities of 

 metabolically incorporated sulfur. 



Rates of Uptake of Radiosulfate by E. Coli. The methods used for the meas- 

 urement of the water space are too slow to show the rate of diffusion across 

 the cell membrane. Some lower limits of the rate can be deduced from the 

 rate at which the cells assimilate sulfur. During exponential growth the quan- 

 tity of sulfur incorporated per gram dry weight of cells per second is 8. i X io~- 

 micromoles. This corresponds to an uptake of 1.2 X lo"* atoms of sulfur per 

 cell per second assuming an average cell volume of io~^- ml. When the sulfur 

 content of the medium is o.i Mg/ml, every o.i second the cell will metabolize 



Table 5. Sulfur exchange in growing e. coli 



.01 mg/ml sulfur as sulfate in all tubes; .01 mg/ml sulfur additional from indicated sup- 

 plements. 



enough sulfur to deplete the water space volume, and, therefore, it must rely 

 chiefly on exogenous sulfur to maintain growth. Since this concentration is 

 sufficient to support growth at optimal rates, it is obvious that the rate of 

 transport of sulfate across the cell boundary is very rapid. 



Though this rate of penetration is rapid, even faster rates are required to 

 supply the sulfur to cells previously grown in sulfur-delicient media as shown 

 by Cowie, Roberts and Bolton (11). Immersion of these cells in a complete 

 medium containing radiosulfate at a concentration of 0.1 Mg/inl produces an 

 initial uptake rate live times as fast. At this rate the cell would sweep out the 

 sulfur of 50 cell volumes of medium per second. The calculated diffusion rate 

 in water would permit 100 times this rate of penetration. It therefore appears 

 that the membrane of E. coli is sufficiently permeable to sulfate ions so that 

 the rate of penetration causes no limitation on the biochemical activities of 

 the cell. Such rates are far too high to be measured by ordinary kinetic methods. 



Sulfate Summary. It is evident that the sulfur content of E. coli is distrib- 



