478 Discussion 



of the cytochrome from its particulate site, or other structural changes resulting from 

 the isolation procedures. The possiblity of either type of change must be considered 

 in assessing the properties of any particulate component removed from its physiological 

 environment, and, on the other hand, the uncertain significance of potential deter- 

 minations on a particulate component in situ must also be kept in mind. 



Henderson: There is doubt as to the validity of £'„ values determined on particulate 

 systems in so far as they apply to some molecular species on or within the particle 

 (see also Paul, The Enzymes, Academic Press, 1951). This is shown e.g. by the results 

 of Baumberger (Cold Spring Harbor Sympos. Quant. Biol. 7, 195, 1939) where the 

 degree to which yeast cells were washed and aged caused a variation from -HO- 12 to 

 +0-31 V in the potential of the medium at which the bands of reduced cytochrome c 

 appeared. Apart from any other factors it would seem rather unlikely that the com- 

 plete thermodynamic reversibility necessary for correct E'q evaluation would exist in 

 a complex particulate system and this would be especially the case with the whole cell. 



Postgate: I think that the absurdity of discussing redox potentials of materials bound at 

 specific sites in cells is illustrated by considering the effect of pH on this quantity. 

 If a bacterium of volume 1 fi^ has an internal pH of 7, a little arithmetic shows that 

 there are only about 60 hydrogen ions to be expected there. The meaning of pH and 

 hence Eh, at specific sites in the cell is even less meaningful. 



