DEAN B. COWIK AND RICHARD B. ROBERTS 9 



uletl among two classes of sulfur compounds which can be clearly distinguished 

 from each other. The first is the bound sulfur which includes the protein sulfur 

 of the cell together with sulfur found in smaller molecules such as glutathione 

 (25). This sulfur content is characteristic of the cells and is, to a limited extent 

 at least, independent of the chemical form and concentration of the sulfur in 

 the medium. In addition to the bound sulfur, the cells are permeated by sulfate 

 from the medium. The concentration of this 'water space' sulfate is always 

 directly proportional to that of the medium; consequently, it is readily removed 

 by washing. 



The incorporation of sulfate from the medium into the bound sulfur of the 

 cells shows all the characteristics of a synthetic process. The quantity incor- 

 porated is proportional to the increase in cell mass, i.e. to the increase of pro- 

 tein. It is, therefore, sensitive to all the factors which influence protein syn- 

 thesis, e.g. the presence of glucose and nitrogen in the medium, temperature, 

 aeration and so on. 



In contrast, the penetration of sulfate into the water space of the cells is 

 independent of: i) the presence of glucose in the medium (tables i, 2); 2) the 

 presence of a nitrogen source (tables i, 2); j) temperature variations (tables i, 

 2, 3); 4) degree of aeration (tables i, 2); and 5) time. These characteristics 

 make it reasonable to believe that no metabolic 'active transport' is involved; 

 diffusion through a permeable membrane is apparently an adequate description. 



Furthermore, independently of the concentration of sulfate in the medium, 

 the pellet of cells contains 75% as much sulfate per ml as is present in the 

 medium. As this amount corresponds roughly to the fluid content of the pellet 

 measured by the loss of weight on drying, the concept of a 'water space' which 

 assumes the sulfate concentration of the medium appears satisfactory. 



The permeability of E. coli to sulfate has been discussed at the onset of this 

 paper because with these cells the distinction between water space sulfur and 

 that metabolically bound is easily demonstrable. Very little sulfate is bound 

 unless the conditions permit growth and, once it is bound, very little sulfur 

 is lost or exchanged. The same methods have been used to study the perme- 

 ability of E. coli and other microorganisms to a variety of materials. In some 

 cases there is more difliculty in separating the water space penetration from 

 metabolic binding and the results are more difficult to interpret. 



Permeability of Torulopsis Utilis to Sulfate Ions. While the results of the 

 studies with E. coli provide a foundation for the design and interpretation of 

 experiments with other microorganisms, species variations often introduce 

 factors which must be taken into account. Torulopsis utilis was chosen to 

 determine whether or not the concept of permeability by passive diffusion 

 could be extended to include other microorganisms besides E. coli. 



Unlike E. coli, T. utilis metabolically incorporates sulfate sulfur without 

 growth. This uptake is rapid, the sulfur being found mainly in the trichloro- 



