394 RESPIRATORY METABOLISM 



tivation of the respiratory mechanism of obligatory anaerobes by O^, — 

 is mentioned by Broh-Kahn and Mirsky (1938), but at present is 

 unsupported by experimental evidence. The bacteriostatic effect of dyes 

 might also be interpreted to mean that these have an inactivating effect 

 on the respiratory mechanism. 



In connection with the effect of oxygen on anaerobes, it should be 

 mentioned that the converse problem also exists. Why do aerobes die in 

 the absence of oxygen.'' On this question there has been considerably 

 less discussion in the literature than on the former. It can be seen from 

 the respiratory equations given above that activation of the substrate 

 and partial oxidation can proceed without Oo. Even in the presence of 

 O, a considerable destruction of substrate, in some forms (e.g., trypano- 

 somes, cited above), seems to be incomplete. Poisoning of the aerobic 

 cell under anaerobic conditions is supposedly caused by the accumulation 

 of toxic products of carbohydrate cleavage or of incomplete oxidation, 

 or by the conversion of all of the respiratory pigment to the reduced 

 state. In facultative anaerobes the former, and in obligatory aerobes the 

 latter theory seems more probable. The observation of Faure-Fremiet, 

 Leon, Mayer, and Plantefol (1929) that Paranieciuni withstands lack of 

 O2 longer at 4° C. than at higher temperatures, is open to either inter- 

 pretation. The data of Piitter (1905), which indicate that 'Paramecium 

 can live longer under anaerobic conditions when the ratio of volume of 

 medium to cells is higher, can be explained on the basis of accumulation 

 of toxic products. What these toxic products are probably depends to a 

 large extent upon the organism, the substrate, and the conditions of the 

 experiment, and the most likely possibilities include lactic and lower 

 fatty acids. 



Oxidation-Reduction Potential Versus Respiration 

 AND Growth 



Another question which arises is why we might or might not expect 

 the oxidation-reduction potential of the medium to affect the respiration 

 and growth of the organism. It is obvious from the outline of the respir- 

 atory processes given above, and from many other types of data, that 

 oxidation-reduction phenomena are involved in respiration. The respira- 

 tory pigments and perhaps also the respiratory enzymes are reversible 

 oxidation-reduction systems. Therefore their action should be affected 



