MefabolJc Pafhways 103 



properties of matter that cells are necessary, are indeed the 

 vital necessity, for life. 



Characferhfic Metabolic Reactions of Cells 



Within this framework of space relations, and as part of 

 its impact upon the nature of cell reactions, the type of 

 reactions characteristically part of the living pattern can 

 be discerned from the studies of metabolic pathways. There 

 are, to my mind, three such types of reactions which are 

 particularly important, since they are characteristic of the 

 living chemistry. The first is the predominance of group 

 transfer. The second is the organization into reaction se- 

 quences of fixed order, some of which are cyclic, others not. 

 The third is the competitive pattern of such reactions or 

 reaction sequences. To the best of my knowledge, these 

 three characteristic metabolic reactions occur in any living 

 cell, and are not unique to the microbe. We shall post- 

 pone, for the moment, a consideration of the important 

 and distinct role that microbiology has played in the ac- 

 cumulation of this knowledge until we have considered 

 the knowledge itself, and where possible, its significance 

 for the understanding of the living process. 



It seems probable to me that a great deal of the chem- 

 istry of the cell is conducted by way of group transfer and 

 exchange. The pattern which I feel is emerging is that 

 there is de novo synthesis of relatively few structural types 

 and that from this stage on, the entire structure or critical 

 portions thereof are moved from one combination into 

 another. The now classical cases of group transfer — methyl, 

 acetyl, amino, guanido, and saccharide groups — are but 

 the beginning; and transpeptidation, transamidation, and 

 the exchange of bases by the desoxyribosides are reflections 

 of a basic metabolic mechanism that we may reasonably ex- 

 pect to be employed in even more complex reactions. 



This mechanism of group transfer would seem to repre- 



