312 A. C. R. Dean and Sir Cyril Hinshelwood 



aerobic to anaerobic conditions, and the presence of toxic sub- 

 stances all demand active adjustments of one kind or another. 



Even if the cells continued to grow in a medium maintained 

 quite constant, adjustments would still be necessary. As the 

 cell size increases, the ratio of area to volume diminishes. The 

 ratio of chemical change per unit volume in the interior to 

 rate of supply of nutrient and rate of removal of products 

 therefore changes so that the metabolic balance is constantly 

 disturbed. The changes are kept within relatively narrow 

 limits by the periodic division of the cell, a process which 

 itself constitutes an automatic adjustment ensuring continued 

 efficient function. 



When cells are transferred to quite new environmental 

 conditions the necessary adjustments are similar to, though 

 they may be considerably more drastic than, those coming 

 into play during the normal growth cycle. 



Kinetic principles of growth 



The available cybernetic mechanisms are chemical in 

 character, and depend upon the fundamental kinetic laws of 

 growth. These have already been discussed on more than one 

 occasion (Dean and Hinshelwood, 1955) and will be summar- 

 ized only very briefly. Growth involves the reproduction of 

 all cell parts. The synthesis and functioning of enzymes must 

 therefore be intimately linked. The reaction pattern of the 

 cell involves a complex set of consecutive and parallel re- 

 actions in which the products of one step are the reactants of 

 the next. Cell division is triggered off by the formation of 

 critical amounts of key substances. Simple and general 

 mathematical treatment then shows that the proportions of 

 the various cell constituents will adjust themselves gradually 

 to such values that an optimum growth rate is established, and 

 will tend to be maintained. If the conditions change, the 

 enzyme proportions will change also. 



The most important theorem here is that these changes take 

 place in such a way as to keep the total growth rate at the 

 highest possible value. An essential part of the kinetic scheme 



