ORIGIN OF THE FIRST ORGANISNfS 377 



literally, all representatives of the living world which have 

 been studied in this respect. 



However, the form of organisation of the chain of processes 

 on which extracellular fermentation is based is still relatively 

 primitive. It is only based on a certain qualitative composi- 

 tion of the mixtures of enzymes, i.e. the obligatory presence 

 in it of the whole collection of enzymes of the zymase com- 

 plex. The sequence of reactions in extracellular fermentation 

 simply depends on each intermediate product having its own 

 specific enzyme. Other transformations of the product are 

 excluded because, in the absence of the corresponding cata- 

 lyst, they would proceed incomparably more slowly than the 

 reaction which is accelerated by the enzyme. For this reason 

 the whole process of extracellular fermentation is of the 

 nature of a straight, unbranched chain. In the living cell 

 it is of great importance not only what enzymes are present 

 but also what are the quantitative relations between the 

 various catalysts acting there ; there must always be a certain 

 correspondence between their activities. This is specially 

 important when one and the same substrate can interact with 

 several of the enzymes present in the cell. As a result of this 

 the substrate is, in fact, altered in different directions. The 

 chain of reactions then becomes branched, and the relation- 

 ship between the rates at which reactions occur in the 

 different branches has sometimes been found to determine 

 whether or not some vital process can take place. A small 

 change in this relationship may cause not merely the cessation 

 of a process, but even the disruption of the whole system. 



As an example of this we may cite the phenomenon of 

 respiration in the plant cell. It only takes place normally 

 when the process of oxidation of the chromogens into respira- 

 tory pigments by the oxygen of the air and the reverse process 

 of their reduction at the expense of the hydrogen of the 

 appropriate donors, correspond very closely with one another, 

 when their rates bear a precisely determined relationship to 

 each other. If, as happens on mechanical injury to the cell, 

 the rate of oxidation is increased disproportionately to that 

 of reduction, the respiratory pigment will not be able to be 

 reduced and will undergo further oxidation into a stable 



