UNITY AND DIVERSITY IN THE METABOLISM OF MICRO-ORGANISMS 



which reveals itself through their requirement for oxygen. The valid- 

 ity of this concept was emphasized when Rubner experimentally es- 

 tablished his principle of 'isodynamic substitution'. He showed that, 

 up to a point, proteins, fats, and carbohydrates can replace one 

 another, in a weight ratio that is inversely proportional to the heat of 

 combustion of these substances, in the adult animal for the mainte- 

 nance of the same vital functions; in other words that, within certain 

 limits, equality in chemical energy corresponds to equality in food value. 



As early as 1885 Rubner pointed out that in all probability it would 

 be possible that also in the metabolism of micro-organisms a conver- 

 sion could be designated which derives its significance for the organism 

 entirely from the resultant energy liberation. Now Pasteur, who, in 

 i860, had discovered the first instance of organisms that can multiply 

 in the complete absence of oxygen, had intuitively recognized the con- 

 nexion between the absence of respiration in these organisms and 

 the fermentation process that characterizes their mode of life. 



Meanwhile the mutual substitution of respiration and fermentation 

 was initially considered largely from a material angle, so that the 

 term fermentation was used to indicate a respiration with bound oxy- 

 gen. The need to consider this substitution primarily on the basis of 

 energetics was first formulated with sufficient clarity by Rubner as a 

 logical consequence of his attempt to interpret the metabolism of any 

 and all living creatures from a single point of view. But not until 1902 

 did he begin his series of micro-calorimetric measurements of the 

 metabolism of various microbes which corroborated the validity of 

 these hypotheses. Since that time it has been satisfactorily established 

 that the metabolic activities of every microbe comprise the processes 

 whereby new cell material is synthesized, as well as dissimilatory pro- 

 cesses characterized by the fact that chemical energy is utilized by the 

 living cell for its energy-requiring functions, and finally appears as heat. 



A number of the most important dissimilatory processes encountered 

 among various groups of micro-organisms is summarized in Table L* 



* In computing the majority of the caloric effects listed in this Table, I have been 

 privileged to profit by the authoritative advice of my colleague, P. E. Verkade, to 

 whom, here too, I want to express my sincere gratitude. It may be mentioned that 

 an attempt has been made to calculate, as accurately as possible, the differences 

 between the heats of combustion of the aqueous solutions of the substrates and of 

 the metabolic products. 



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