MICROBE AND LIFE 



olism had gradually gained ground, there were many who felt attracted 

 to a more profound study of these phenomena. I shall restrict myself 

 to indicate a few main lines along which our perception has increased 

 as a result of these investigations. 



In the first place, the various and divergent energy-providing pro- 

 cesses have been subjected to a closer analysis. The respiratory pro- 

 cess, during which some carbohydrate reacts with oxygen and is here- 

 by converted into carbon dioxide and water, was one of the first pro- 

 cesses into which a more profound insight was gained. In view of the 

 fact that, at the low temperatures prevailing in our biosphere, sugars 

 do not react at all with oxygen, it is highly remarkable that this pro- 

 cess occurs quite intensively in the interior of many living cells. Per- 

 tinent investigations have shown that the glucose molecule undergoes 

 a long series of transformations, the most fundamental among which 

 are undoubtedly those reactions in which hydrogen atoms are removed 

 from one of the intermediate products, and, under the influence of a 

 large number of interposed catalysts, are transferred with the ultimate 

 formation of a compound that can spontaneously react with free 

 oxygen. In other words, the intermediate products formed from the 

 sugar function as donators of hydrogen atoms, for which ultimately the 

 oxygen serves as acceptor. 



Now the concept thus developed, viz., that the most common type 

 of respiration is essentially based on a catalytic transfer of hydrogen, 

 has turned out to be the key to an understanding of the numerous 

 other energy-providing processes. Soon a similar situation was also 

 encountered in those cases where substances other than sugars serve 

 as the respiratory substrates. These, too, appeared to function directly 

 or indirectly as donators of hydrogen atoms that ultimately found 

 their way to oxygen. Hence this means that a single fundamental 

 principle lies at the root of the diversity of metabolic phenomena ex- 

 hibited by respiring micro-organisms that is at first sight so confusing. 

 In the course of time life has apparently adapted itself to quite a vari- 

 ety of hydrogen donators, thus enormously extending the range over 

 which it can exist. 



But even this was not the end of it. I have already mentioned that 

 in the microbial world there also occur numberless types with mutually 

 extremely divergent metabolic patterns that do not require free oxy- 

 gen for their perpetuation, nay, whose development may even be in- 



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