346 PROTOPLASM 



the relative activity of these two processes in the cell. With a 

 definite, limited series of indicators, a point on the scale can be 

 found that will indicate the potential (if the system is in equilib- 

 rium) or, if not that, then at least the relative rates of oxidation 

 and reduction. 



Thus do dyes indicate the reduction potential of tissues by 

 color changes analogous to those which indicate pH values. 

 (The electrometric method has so far proved impossible for 

 studies on cells.) In some cases the cells themselves may con- 

 tain their own naturally occurring reduction indicator, such 

 as echinochrome in Echinoderm eggs, and cytochrome in yeast 

 and other cells. 



Oxidation-reduction potentials were formerly expressed in 

 terms of rH, a symbol devised by W. M. Clark to express similarity 

 between oxidation-reduction potentials and potentials set up 

 by hydrogen ions. The analogy with pH was shown by defining 



rH as log ^ An oxidation-reduction potential of 



II2 pressure ^ 



0.81 would have an rH of 41; and a potential of 0.072, an rH 



of 11.6. As the analogy is not complete, the symbol has been 



dropped. 



Apparently, Gillespie was the first to measure an oxidation- 

 reduction potential produced by organisms (cultures of Bacterium 

 coll). W. M. Clark began his extensive studies with an investiga- 

 tion of the reducing power of milk. Needham was the first to 

 apply the microinjection method to studies on oxidation, which 

 have been extended by Rapkine and Wurmser and particularly 

 by Cohen and Chambers. M. M. Brooks employed the immer- 

 sion method by bathing cells in dyes which enter readily; the 

 cell sap of the alga Valonia was found to have a potential of 

 -1-0.12 to -f0.15 volt (an rH of 16 to 18); and the protoplasm, 

 a potential of 0.21 to 0.48 volt. Many potential studies have 

 been made of bacteria and yeast, though obviously not of the 

 interior of the cell itself but merely of the surrounding 

 solution. 



It is, therefore, necessary to differentiate between potentials 

 obtained within cells and those obtained in cell suspensions. 

 Potentials in cell suspensions must be referred to cellular products 

 which have passed into the suspension medium. Values for 

 cell suspensions have, as a rule, been obtained by the electro- 



