28 ELECTROLYTES IN BIOLOGICAL SYSTEMS 



DISCUSSION AND CONCLUSIONS 



In these experiments with E. coli we have found that when cells are immersed 

 in radioactive solutions and then centrifuged, the pellet of cells invariably 

 contains a predictable minimum quantity of radioactivity. After correction 

 for the radioactivity which is metabolically bound, the residual quantity taken 

 up per gram of wet cells is equal to the radioactivity of 0.75 ml of the immersion 

 fluid. The only exceptions noted have been where labeled proteins were used 

 and in this case the quantity taken up per gram was equal to the radioactivity 

 of 0.1 to 0.2 ml of medium. x\s the value 0.75 ml per gram of wet cells is equal 

 to the loss of water observed when the pellet is dried, it appears that the water 

 contained in the pellet has the same concentration of dissolved radioactivity 

 as does the original immersion fluid. In other words, the pellet of cells has a 

 'water space' of 0.75 ml per gram wet cells and the 'water space' has the same 

 concentration of small ions and molecules as the medium. 



A part of this water space is undoubtedly due to the fluid trapped between 

 the cells of the pellet, the intercellular fluid. The volume of this intercellular 

 fluid can be measured by the quantity of radioactive protein held in the pellet 

 after immersion of cells in medium containing labeled proteins. Our values 

 give only an upper limit of this volume as a part of the protein might be held 

 on the surface of the cells or might even penetrate the cells. Even so the inter- 

 cellular fluid volume is less than one-third the total water space, and the re- 

 mainder must be within the cells. 



The material which we consider as dissolved in the water space could not 

 be adsorbed on the surface of the cells. The same apparent volume is found 

 for a wide variety of ions and molecules; it is independent of the concentration 

 in the medium, and is numerically equal to the actual water content of the 

 pellet measured by drying the cells. It therefore appears that a wide variety 

 of ions and molecules penetrate the cell wall in either direction within a few 

 minutes and are present within the cellular water space at the same concen- 

 tration as in the surrounding fluid. This penetration does not depend on 

 metabolic activity and appears to be due solely to diffusion through a permeable 

 membrane. Similar conclusions can be reached by considering the rates at 

 which exogenous metabolites penetrate to the sites of enzymatic activity and 

 cause immediate changes in biochemical pathways. 



The rate of penetration is too rapid to be measured by our methods. Equilib- 

 rium between the outside and inside concentrations is reached within 5 minutes. 

 Exogenous metabolites show their influence in synthetic activities within 30 

 seconds. In any event the cell wall is sufficiently permeable so that the penetra- 

 tion is not one of the rate limiting factors in biochemical reactions. 



The substances for which permeability has been measured directly by ob- 

 serving the water space content include: the cations Na+, K+, Rb+, and Cs+; 



