DKAX H. t'OWIF. AND KICHAKI) H. KOBKRTS 29 



1 he anions S04^ and P(^4°; the organic compounds glucose-i-phosphate, fruc- 

 tose- 1 :6-phosphate, fructose, glutamic acid, methionine, cystine, and gluta- 

 thione. The permeability has been shown indirectly by observing the rapid 

 intlucnce on synthetic activities exerted by many of the common metabolites 

 imluding glucose, homoserine, threonine, asparlic acid, isoleucine, ornithine, 

 citrulline, arginine, proline, leucine, glycine, a-ketoglutyric acid, a-kelomethyl- 

 valerate, adenine, guanine, cytodine, uracil and many others. Similar but less 

 extensive observations using T. u/ilis have not indicated any lack of pernie- 

 ability in this organism. 



The E. coli membrane therefore is a morphological boundary within which 

 are assembled the reactive centers of metabolism. The protoplasm of the cell 

 is in direct contact with the environment, and the kinds of biochemical reac- 

 tion which occur reflect this intimate association. The protoplasm may be 

 likened to a sponge, the cell membrane to a surrounding hair net unable to 

 exclude the entrance or emergence of small molecules. Into this system water 

 may diffuse freely and carry with it many of the dissolved substances which 

 it contains. Thus the nutrients of the environment dififuse into the reactive 

 sites of the protoplasm and become nondiflfusible. 



Not all microorganisms are permeable to small molecules. Mitchell (i8) has 

 shown that an osmotic barrier near the external surface of Micrococcus pyo- 

 genes is impermeable to inorganic phosphate ions. Conway and Downey (7) 

 find that only 10 to 11 % of the total cell volume of 'Baker's yeast' is permeable 

 to sodium and potassium chloride. Similar results were obtained with arabinose, 

 galactose, succinic acid, glyceric acid, and a number of other acids with hy- 

 droxyl or amino groups. These findings also support the concept of an im- 

 permeable barrier near the external surface of these cells. 



On the other hand, these authors show that the yeast cell is freely permeable 

 to formic, acetic, propionic, and butyric acids. Maass and Johnson (16) using 

 S. cerevisiae find that these cells are impermeable to penicillin, but that, with 

 Micrococcus pyogenes, a simple diffusion of penicillin occurs so that the intra- 

 cellular water has the same penicillin concentration as the extracellular water. 

 Williams and Wilson (32) have shown that in Azotobacter, succinate removed 

 from external solutions was proportional to the concentration of succinate in 

 the medium when the medium was maintained at 3°C. The same result was 

 observed for succinate adapted and unadapted cells. 



Species variations thus seem to be an important factor in the permeability 

 of cells to small molecules. However, even in the same species variations may 

 exist. The data presented in this manuscript were obtained over a four-year 

 period and were always consistent. Recently, variations in the 'sulfate space' 

 of several strains of E. coli have been observed. The values have ranged from 

 80 to 30% of the cellular volume. The culture conditions which alter the cell 



