424 FURTHER EVOLUTION 



co-ordinated reactions, each of which is catalysed by its own 

 specific enzyme. Thus, this exchange of energy demands a 

 rather highly developed internal chemical mechanism, and, 

 the longer the chain of reactions, the more co-ordinated the 

 mechanism must be. 



It would be theoretically possible to imagine an infinitely 

 large number of such chains of energy exchange, each 

 different in principle from the others both as regards the 

 individual reaction-links and as regards the general structure 

 of the whole chain. It is therefore very remarkable that 

 extensive biochemical researches have established the fact 

 that in all organisms which have yet been studied in this 

 respect, the energy metabolism is based on extremely similar, 

 almost identical, systems of reactions, catalysed by identical 

 enzymes. The system may vary from organism to organism, 

 but only in detail ; if one enzyme is absent another takes 

 its place, but, as a whole, it seems to be the same throughout 

 all the stages of evolutionary development of all the inhabi- 

 tants of the Earth, both anaerobes and aerobes. Among 

 aerobes new catalytic mechanisms have been added to the 

 original system, enabling them to use molecular oxygen. 



To familiarise ourselves with the actual working of the 

 basic system we may consider the chemical mechanism of 

 alcoholic fermentation, which has now been thoroughly 

 studied. It takes place in a number of micro-organisms, of 

 which yeast would seem to be the most typical. The general 

 scheme of the reactions of fermentation as given by V. L. 

 Kretovich in his book,^° is shown in Fig. 36. The diagram 

 shows that glucose is transformed into ethyl alcohol and 

 carbon dioxide, without the participation of molecular 

 oxygen, by means of a series of strictly co-ordinated enzymic 

 reactions. 



The process starts with the phosphorylation of glucose 

 with the help of the enzyme hexokinase. This involves the 

 transfer by hexokinase of a phosphate residue with a high- 

 energy bond (Af approx. 8,000 cal/mole) from ATP to a 

 glucose molecule. It leads to the formation of glucose-6- 

 phosphate and adenosine diphosphate (ADP). The glucose-6- 

 phosphate is then transformed into fructose-6-phosphate by 

 the enzyme oxoisomerase. The fructose-6-phosphate combines 



