378 THE FIRST ORGANISMS 



brown pigment which cannot serve as a hydrogen acceptor. 

 In this way all the chromogen of the cell is very quickly 

 converted into an inactive state and the process of respiration 

 ceases as a result of the disturbance of the mechanism on 

 which it is based.^^ 



There is a great variety of substances in protoplasm by 

 means of which the accurate regulation of the catalytic activ- 

 ities of the enzyme complex is accomplished. In addition to 

 the new formation and irreversible destruction of the proto- 

 plasmic enzymes, there also occurs a widespread reverse 

 activation or inhibition of these catalysts. The protein nature 

 of enzymes not only determines their exceptional activity 

 and the specificity of their effects, it also determines their 

 great lability, their extreme sensitivity to different kinds of 

 physical and chemical factors. Any rough treatment will 

 cause the denaturation of proteins and their catalytic activity 

 will be irreversibly lost. But by treatment which does not 

 lead to denaturation the activities of enzymes may be altered 

 reversibly over a very wide range. In fact there is no physical 

 or chemical factor, no organic compound or inorganic salt, 

 which cannot affect the course of enzymic reactions in one 

 way or another. Any raising or lowering of the temperature, 

 any change in the acidity of the medium, its oxidation- 

 reduction potential, its salt content or its osmotic pressure, 

 interferes with the relationship between the rates of the 

 different enzymic reactions and thus changes their inter- 

 connections in the network of metabolism. Of great import- 

 ance in this connection is the development among the com- 

 ponents of protoplasm of various activators and inhibitors 

 with specific activities, which selectively speed up or slow 

 down any one or several enzymic reactions.^® 



Owing to the action of all these supplementary chemical 

 mechanisms which are intimately associated with the physico- 

 chemical state prevailing at any given moment within the 

 protoplasm, very precise quantitative relationships are estab- 

 lished between the rates of the enzymic reactions. These 

 relationships may, however, vary greatly both as between 

 different organisms and even in a single cell at different 

 periods of its existence, and owing to the effects of different 

 external and internal conditions. This gives a form of organ- 



