PHYSICO-CHEMICAL ASPECTS I5 



It is very important to notice that as a result of the pre- 

 sence of granules, chromosomes, membranes, nucleoli 

 and other formed bodies in the cell, there must necessari- 

 ly be great importance attributed to surface properties. 

 The main molecular constituents of cells, the proteins 

 and nucleic acids, are themselves such large molecules 

 that any reaction taking place with them, or in which 

 they are involved, is necessarily a surface action and not 

 a bulk reaction. And it must very often be the case that 

 two reacting molecules must be regarded as reacting in 

 the zone constituted by their overlapping surfaces. In 

 considering the basic physico-chemical nature of the 

 cell, we must therefore be particularly alive to the im- 

 portance of surface properties. 



The Control of Enzyme Systems. Amongst the most im- 

 portant of the physico-chemical systems which form an 

 integral part of living cells are the systems controlling 

 the activity of intracellular enzymes. Among the factors 

 involved in controlling enzymes are hydrogen ion con- 

 centration, the concentration of SH groups, the concen- 

 trations of inhibitors and activators, and those processes 

 which control access of substrates to enzymes. 



The hydrogen ion concentration is maintained con- 

 stant in cells partly by the buffering substances normal- 

 ly present, and partly by the active excretion of excess 

 of acidic or basic substances. The SH content of a cell 

 is important because many enzyme systems have maxi- 



