866 GENERAL METABOLISM [pt. hi 



as opposed to the total quantity of oxidants or reductants present. 

 These conceptions apply, strictly speaking, only to systems which are 

 reversible, while the living cell is a complex association of oxidation- 

 reduction systems, some of which are probably reversible, and some 

 of which are not. Nevertheless, the application of the conception 

 of oxidation-reduction potential to the processes occurring in the 

 living cell has justified itself by its results. The living cell cannot be 

 thought of as a simple reversible system, and true equilibrium must 

 be sharply distinguished from a steady balanced state maintained 

 at its characteristic level by the velocities of a chain of reactions. 

 The latter condition is what is found in the living cell, which is 

 balanced very steadily at its characteristic level of oxidation- 

 reduction intensity. Between the entering hydrogen acceptor, oxygen, 

 of high rH, and the reducing systems such as glutathione and 

 xanthine oxidase, of low rH, the cell maintains its overall rH closely 

 around oxidation-reduction neutrality in ordinary aerobic conditions. 

 These interesting problems cannot be treated here in detail, but 

 a full account of them will be found in the series of experimental 

 papers due to Mansfield Clark and his collaborators. The reviews 

 of Clark; Dixon; Conant; Michaelis and Wurmser deal with the 

 theoretical aspects of the application of rH to biological problems, and 

 the review of Needham & Needham should be consulted for an account 

 of what has actually been done in this direction. It must suffice to 

 say here that, just as ihepH is the negative logarithm of the hydrogen 

 ion concentration, so the rH is the negative logarithm of the pressure 

 of hydrogen gas in a platinum electrode in equilibrium with the 

 given system, and to note that the behaviour of strong and weak 

 acids and bases, buffers, indicators, etc., all find a counterpart in 

 oxidation-reduction equilibria. 



The earliest determinations of the rH of biological systems by 

 means of rH indicators, i.e. dyes whose oxidation-reduction potentials 

 at all stages of decolorisation to the leuco-bases had been accurately 

 ascertained in vitro, were made by Clark. In 1 925 my wife and I began 

 a series of experiments in which these indicators (both completely 

 oxidised and completely reduced) were micro-injected into single cells. 



As regards the rH of the egg-cell, we reported that, in Strongy- 

 locentrotus lividus, Asterias glacialis, Ophiura lacertosa, Echinocardium 

 cordatum, Sabellaria alveolata, and Ascidia mentula, the aerobic intra- 

 cellular rH was always between 19 and 22. Later, Chambers, 



