106 Perspecfives in Microbiology 



self-limiting reactions and series inhibition, which require 

 that the cell operate its reaction pathways essentially one 

 molecule at a time. This molecule-at-a-time requirement 

 permits the free play of the third characteristic factor of 

 the living process, that of competition. There are in every 

 cell alternative pathways that may be followed by a given 

 molecule. Whether a given molecule of glucose-6-phos- 

 phate is converted to fructose-6-phosphate, to phospho- 

 gluconic acid, to glucose- 1 -phosphate, or to glucose plus 

 phosphate, which step determines its subsequent fate, 

 depends on circumstances within the cell. 



That such competition may be the controlling factor in 

 cell growth and development was forcibly impressed upon 

 us in studies on the mode of action of streptomycin. Here 

 the resting cells possessed what appeared to be a typical 

 citric acid cycle, and it was entrance into this cycle that 

 was inhibited by streptomycin. Further study showed that 

 the citric acid cycle was indeed present but that it was not 

 sensitive to streptomycin. Rather, there existed side by 

 side with the citric acid cycle a hitherto unknown reaction 

 sequence, the first step of which was the formation of a 

 seven-carbon phosphorylated intermediate. This alterna- 

 tive sequence apparently started from the same materials, 

 was in competition with the citric acid cycle for these sub- 

 stances, and led eventually, but by different paths, to the 

 same carbon dioxide and water. Along the alternative 

 route, however, there must be something that the cell re- 

 quired, since this alternative pathway was originally recog- 

 nized only by its inhibition by streptomycin, and since, 

 when it was inhibited, the cell could not grow. 



It seems to me, therefore, that the studies of metabolic 

 pathways have told us the following things about the living 

 process in whatever cells we may find it: First, that the re- 

 actions occur at and because of specific surfaces. Second, 

 these reactions are characterized by: (a) predominance of 

 group transfer; (b) organization into reaction sequences 



