RESPIRATORY METABOLISM 395 



by the potential of the medium in which they are found, that is, by the 

 oxidation-reduction potential of protoplasm, and they, in turn, must 

 to a large extent determine this potential. If we were to mix numerous 

 half-reduced, completely reversible oxidation-reduction indicators in a 

 homogeneous solution, an equilibrium would be reached, and the po- 

 tential attained would depend upon the £„ values of the substances and 

 upon their relative amounts. Substances with Eo values far from the result- 

 ing Eh value of the solution would be either completely reduced or com- 

 pletely oxidized, and would be unable to contribute much toward oxidizing 

 or reducing small amounts of added materials unless the Eu of the mixture 

 were appreciably changed. 



Obviously such a simple system is not present in protoplasm. Because 

 of the presence of irreversible systems and of the continual introduction 

 of new substrate and the removal of certain end products, a true equi- 

 librium is never attained. Also, the colloidal nature of protoplasm makes 

 possible the existence of different Ei, values in different phases of the 

 substance, and the differential adsorption of oxidized and reduced ma- 

 terial at interfaces may produce a potential different from that in any of 

 the phases. Therefore the term "oxidation-reduction potential of proto- 

 plasm" may be without any interpretable significance (for discussion, see 

 Jahn, 1934; Korr, 1938). But there must certainly be a significance to 

 the Eo values of the respiratory pigments, and the possibility of an indi- 

 vidual expression of these values may be maintained by the polyphasic 

 nature of protoplasm. The oxidation-reduction potential, which can be 

 measured with indicators, is probably an index of the potential developed 

 by one or more of these pigments (for summary of such measurements, 

 see Chambers, 1933; Cohen, 1933). It is known that the apparent Ei> 

 value of protoplasm, as measured by indicators, varies with the Eh of the 

 external medium when the external O., tension is changed. Therefore, 

 why cannot the Ei, of the external medium determine the degree of re- 

 duction of the respiratory pigments and therefore the rate of respiration.'' 

 This mechanism might be used to explain the inhibition which is pro- 

 duced by oxidation-reduction indicators in cultures of bacteria (Dubos, 

 1929) and in cultures of Chilomonas (Jahn, 1933b). One difficulty in 

 predicting what reactions would occur in protoplasm, even if we had a 

 thorough knowledge of the oxidation-reduction systems involved, is the 

 fact that such knowledge can tell us only what reactions might or might 



