396 RESPIRATORY METABOLISM 



not occur if all of the reactions were reversible. Since many of the re- 

 acting substances are changed irreversibly and since the rates of reactions 

 are dependent not only upon Eo values but upon enzymes, knowledge of 

 Eo and Eh values cannot indicate what reactions will occur. 



Much of this discussion of oxidation-reduction potentials is pure 

 speculation, but it is the type of speculation (often unexpressed) which 

 has spurred investigators to a study of the naturally occurring oxidation- 

 reduction systems, of the apparent oxidation-reduction potential of proto- 

 plasm, and of the Eh values developed in bacterial and protozoan cultures 

 (reviews, Needham and Needham, 1927; Wurmser, 1932; Chambers, 

 1933; Clark, 1934; Hewitt, 1936). Interpretations of the data have not 

 always been as fruitful as one might expect, and one is led at times to 

 suspect that the modes, if not the points of attack on the problem, are in 

 need of revision. However, since the necessity of some such relationship 

 as outlined above seems sound, it is more probable that merely the time 

 for the harvest has not yet arrived. Clark (1934) estimated that another 

 half century will be necessary for the solution of these problems. 



Another means by which the oxidation-reduction potential of the 

 medium is supposed to affect metabolism is described by the surface catal- 

 ysis theory suggested by Quastel (1930), Kluyver (1931), and others 

 (discussed by Hewitt, 1936). It is suggested that many oxidative pro- 

 cesses of bacteria take place at the surface of the cell (Quastel, 1930) , and 

 it seems as if for these reactions the Eh of the medium would be more im- 

 portant than that of the protoplasm. It is also very probable that oxida- 

 tion-reduction enzymes are merely surface catalysts, which produce their 

 effect by nature of intense interfacial electrical fields (Kluyver, 1931). 

 These fields might be affected by the potential of the medium, whether 

 they occur at the cell surface or within the protoplasm. An intriguing 

 speculation would be that the respiratory pigments are distributed among 

 the various phases of protoplasm, and that the enzymes are actually the 

 interfaces of the emulsion. Unfortunately, such ideas are difficult to 

 check experimentally. However, since oxidation-reduction enzymes are 

 proteins which in all probability exert their catalytic properties through 

 surface action, it is possible that the catalytic interfaces of the cells, dis- 

 cussed by Kluyver (1931), are merely the surfaces of the protein mole- 

 cules. 



