CHEMICAL ASPECTS OF LIFE HOPKINS 137 



geneous environment the right material for the maintenance of its 

 structure and activities. It is, however, no vital act but the nature 

 of its specific catalysts which determines what it effectively " selects." 

 If a molecule gains entry into the cell and meets no catalytic influence 

 capable of activating it, nothing further happens save for certain 

 ionic and osmotic adjustments. Any molecule which does meet 

 an adjusted enzyme cannot fail to suffer change and become directed 

 into some one of the paths of metabolism. It must here be remem- 

 bered, moreover, that enzymes as specific catalysts not only promote 

 reactions, but determine their direction. The glucose molecule, for 

 example, though its inherent chemical potentialities are, of course, 

 always the same, is converted into lactic acid by an enzyme system 

 in muscle but into alcohol and carbon dioxide by another in the yeast 

 cell. It is important to realize that diverse enzymes may act in suc- 

 cession and that specific catalysis has directive as well as selective 

 powers. If it be syntheses in the cell which are most difficult to pic- 

 ture on such lines, we may remember that biological syntheses can be, 

 and are, promoted by enzymes, and there are sufficient facts to jus- 

 tify the belief that a chain of specific enzymes can direct a complex 

 synthesis along lines predetermined by the nature of the enzymes 

 themselves. I should like to develop this aspect of the subject even 

 further, but to do so might tax your patience. I should add that 

 enzyme-control, though so important, is not the sole determinant of 

 chemical organization in a cell. Other aspects of its colloidal struc- 

 ture play their part. 



Ill 



It is surely at that level of organization, which is based on the 

 exact coordination of a multitude of chemical events within it, that 

 a living cell displays its peculiar sensitiveness to the influence of 

 molecules of special nature when these enter it from without. The 

 nature of very many organic molecules is such that they may enter 

 a cell and exert no effect. Those proper to metabolism follow, of 

 course, the normal paths of change. Some few, on the other hand, 

 influence the cell in very special ways. ^Vllen such influence is highly 

 specific in kind, it means that some element of structure in the en- 

 trant molecule is adjusted to meet an aspect of molecular structure 

 somewhere in the cell itself. We can easily understand that in a 

 system so minute the intrusion even of a few such molecules may so 

 modify existing equilibria as to affect profoundly the observed 

 behavior of the cell. 



Such relations, though by no means confined to them, reach their 

 greatest significance in the higher organisms, in which individual 

 tissues, chemically diverse, differentiated in function and separated 

 in space, so react upon one another through chemical agencies trans- 



