l6 THE CELL AS A PHYSICO-CHEMICAL UNIT 



mal activity when either in the reduced state or in the 

 oxidised state. Usually the reduction can be carried out 

 by SH compounds, particularly by glutathione. Simi- 

 larly, the reduced forms of enzymes can usually be oxi- 

 dised with the oxidised form of glutathione. Thus the 

 presence within cells of glutathione constitutes a poising 

 system tending to maintain a given degree of reduction 

 of the enzyme systems of the cell, in just the same way 

 as the pH buffers tend to maintain a given degree of ioni- 

 sation of the enzyme systems. 



Equally important with pn and SH content are the 

 factors of inhibition, activation and substrate access. But 

 extremely little is known of the variables which control 

 the operation of these factors. 



However, the cell does not present a completely uni- 

 form environment with respect to pn and SH content. 

 Thus, when we consider a gene present in the nucleus, 

 one of the most striking properties is the high concen- 

 tration of ionising groups in the gene. As a result there 

 is a Donnan equilibrium between the nuclear sap and 

 the gene. Thus the pH in the gene will not be the same 

 as that in the nuclear sap. Even when we are dealing 

 with small bodies, like protein and nucleic acid mole- 

 cules, we encounter local variations in p^. This is be- 

 cause there are high concentrations of ionising groups in 

 the surface of proteins and nucleic acids, so that equili- 

 bria analogous to the Donnan equilibrium exist between 

 the fluid medium of the cytoplasm and the surfaces of 



