366 THE FIRST ORGANISMS 



intermediate product which may be formed within the proto- 

 plasm, will be changed during metabolism only in the direc- 

 tion in which it can react most quickly. All the rest of the 

 reactions, which take place more slowly, will simply not have 

 time to occur to any significant extent. It is in this way that 

 there are formed those strictly determined chains and cycles 

 of successive quick reactions which together constitute the 

 more or less ramifying network of metabolism. 



A simple homogeneous mixture of organic substances, or 

 even a newly formed coacervate drop which has not yet been 

 transformed into a well-organised open system, presents, from 

 this point of view, a very wide but completely untrammelled 

 field of chemical possibilities. The same great difficulties and 

 obstacles hinder movement in any direction in this field. 

 In contrast to this, selection has led to the presence in proto- 

 plasm of definite paths of biochemical processes, a whole 

 network of ' rationally built roads ' along which there pro- 

 ceeds at a great rate and in ' orderly columns ' the chemical 

 transformation of substances and the associated conversion 

 of energy. 



This highly-developed order, which depends on the definite 

 relationships between the velocities of the reactions, is regu- 

 lated in the living body by many factors. The most important 

 of these is the catalytic activity of the enzymes. 



Nowadays the study of enzymes, enzymology, has grown 

 into an extensive and independent field of knowledge in 

 which an immeasurable amount of work is being done.^* 

 Many enzymes have now been isolated from living organisms 

 in the form of highly purified crystalline preparations-^ which 

 have been studied in detail as regards both their chemical 

 nature and the mechanism of their catalytic activity.^® 



These enzymes have been found, without exception, to be 

 simple or conjugated proteins. The prosthetic groups of the 

 latter consist, in most cases, of organometallic compounds or 

 various vitamins. There can now be no doubt that each cell 

 contains a whole collection of diverse enzymes and that the 

 majority of the proteins of the living body have enzymic 

 activity. Thus enzymes would seem to constitute the bulk 

 of the proteins of protoplasm. ^^ 



