ENERGY METABOLISM 419 



Different forms of energy metabolism. 



It would seem, at first glance, that the position is reversed 

 with the second cardinal thesis which we enunciated earlier, 

 that is, the primary nature of the anaerobic degradation of 

 organic substances. Only a veiy limited number of species 

 of bacteria and other lower organisms are obligate anaerobes, 

 living out the whole of their life cycles in the absence of 

 molecular oxygen. Other micro-organisms, such as yeasts, 

 are facultative anaerobes. But the overwhelming majority 

 of contemporary living things, especially all higher plants 

 and animals, cannot do without the free oxygen of the atmo- 

 sphere. 



This state of affairs is highly significant because, under the 

 oxidising conditions of the present time, it is quite possible 

 to oxidise organic substances completely to carbon dioxide 

 and water, which mobilises a far greater amount of energy 

 than the simple anaerobic degradation of these substances. 

 It is therefore quite natural that contemporary living things 

 should, during the course of their prolonged evolution, have 

 become widely adapted to the most extensive use of the 

 conditions which prevailed on the surface of the Earth after 

 a considerable amount of free oxygen had been formed, i.e. 

 from about 700 million years ago. 



It must, none the less, be admitted that anaerobiosis is the 

 primary way of life ; for a careful study of the energetics of 

 metabolism in the most diverse organisms, both lower and 

 higher, has shown convincingly that everywhere (even among 

 aerobes) this metabolism is based on strikingly similar and 

 completely universal anaerobic reactions of degiadation of 

 organic substances, while the very variegated mechanisms 

 which catalyse the combination of molecular oxygen with 

 the products of this degradation in different living things 

 are only superimposed on this basis. 



This state of affairs was noticed as early as the end of last 

 century as a result of purely physiological investigations. 

 It was E. Pfliiger*^ who first discovered the so-called anaerobic 

 respiration of higher animals and put forward the view that 

 this process was not pathological and that it was not a minor 

 biological adaptation to enable organisms to survive a short 



