METABOLIC PROCESS PATTERNS 



Technologically there are numerous disadvantages in operation 

 of the anaerobic type of metabolism. Most prominent is low efficiency. 

 The probable upper limit of the gross yield is about ten per cent, 

 whereas ninety per cent of the potential energy of combustion remains 

 unused in the waste products. A piling up of waste products, fre- 

 quently of strongly acidic character, presents a further serious technical 

 problem. In the more highly organized living systems, therefore, we 

 find the aerobic type predominant. If in the higher organisms we 

 meet, as we do sporadically, a well-developed system of anaerobic 

 energy conversion, it is in places or stages of development at which, 

 for structural or topographical reasons, the oxygen supply is poor or 

 unsafe: in the embryo, in cancer tissue, in parts of the placenta, parts 

 of the retina, in muscle, etc. An anaerobic energy supply grants 

 greater independence (8). 



Process Characteristics of Respiration 



The introduction of oxygen as hydrogen acceptor increases 

 considerably the complexity of the energy-yielding process. Our 

 present insight in this case is spotty and far removed from the com- 

 pleteness achieved in understanding simpler anaerobic fermentations. 

 An attempt has been made here to coordinate the available data into 

 a coherent process scheme, and at the same time to point out those 

 stretches in the flow lines for which information is still missing. 



When, as in Figure 1 , the progressive catabolism of a substrate 

 molecule in the manner of a flow chart is projected onto an energy- 

 time coordinate system, some representative features emerge. The 

 particulars of this scheme refer to degradation of half a glucose unit 

 through the citric acid cycle. The gross energy available from this 

 process, calculated by summation of the areas above the six consecutive 

 steps of dehydrogenation is 6 X 57, = 342 kcal., a quantity practically 

 identical with the theoretical yield for carbohydrate combustion. The 

 dehydrogenation potentials of the intermediaries oscillate almost 

 symmetrically around the hydrogen potential. 



Contrary to present convention, the hydrogen potential at pM 7 is made 

 the reference potential, which coincides with the potential of the "average" respira- 

 tory hydrogen donor. By plotting in a conventional manner oxidation-reduction 

 potentials, E'o, as the difference between the normal potential of the system at 

 pH 7 and the potential of atmospheric hydrogen gas at pH 0, a meaningless zero 



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