426 FURTHER EVOLUTION 



expense of the alteration of the whole of that half of the 

 glucose molecule. 



The high-energy phosphate residue thus formed is trans- 

 ferred to ADP and the second molecule of ATP is regener- 

 ated. The enolpyruvic acid then goes over to its more stable 

 form, pyruvic acid. The enzyme carboxylase splits off a 

 molecule of CO2 from the pyruvic acid and the acetaldehyde 

 so formed is reduced to ethyl alcohol by combining with the 

 hydrogen from coenzyme I. 



How great is the amount of energy obtained by the 

 fermentation of a whole molecule of glucose? 



As we have seen, the transformation of half a glucose 

 molecule (glyceraldehyde-3-phosphate) gives two high-energy 

 bonds which serve for the regeneration of the two molecules 

 of ATP which were used to phosphorylate the glucose. The 

 high-energy bonds derived from the second half of the glucose 

 molecule are a pure energy ' profit ' to the cell and can be 

 used for the synthesis of living matter or for other purposes. 



Thus alcoholic fermentation is a process of anaerobic 

 breakdown of the glucose molecule, in which the energy 

 liberated by dehydrogenation accumulates in the form of 

 the high-energy bonds of phosphate residues and is carried 

 over in this form into the general metabolic system of the 

 cell through the agency of derivatives of adenylic acid, ADP 

 and ATP, with the help of the appropriate enzymes. 



On considering the mechanism of alcoholic fermentation 

 one is struck by the large number of stages involved. The 

 reason for this is that it allows more effective use to be made 

 of the energy liberated by the breakdown of the sugar 

 molecule and also allows the formation of those fragments of 

 molecules from which the organism builds the carbon skele- 

 ton of its living material. 



We find this ability to acquire energy by the anaerobic 

 degradation of organic substances by many stages in all the 

 different systematic groups of organisms, from the most 

 primitive bacteria to the highest mammals. Hence the 

 energy metabolism is based on anaerobic dissimilation of 

 carbohydrates similar to the process of alcoholic fermentation 

 which has just been expounded. Individual links in the 

 chain may vary and accessory superstructures may be elabor- 



