KINETICS OF GROWTH 99 



forming a number of fundamental biochemical syntheses with isolated 

 enzyme systems. 



A special corollary of Kluyver's concept is that the molecular struc- 

 ture of the substrate and of the intermediate products to which this can 

 lead during its breakdown, rather than the amount of energy liberated 

 during this process, becomes the all important determinant of its po- 

 tentialities as the starting point for the synthesis of cell materials. The 

 investigations by Clifton and Logan, Doudoroff, and Bernstein on the 

 assimilation of chemically related compounds at different energy levels 

 (33-36) provided suggestive results in this connection. Better yet can 

 this situation be illustrated by recent studies on the synthesis of some 

 characteristic cell constituents. 



Having recognized a number of vitamins as parts of enzyme systems, 

 it has become increasingly clear that the biosynthesis of such entities 

 proceeds in a chemically intelligible manner, even though many of the 

 steps involved in their manufacture may still be obscure. The mere fact 

 that certain microorganisms are unable to develop in simple media 

 without being supplied with preformed parts of enzymes shows con- 

 clusively that the supply of energy is not the governing factor, and that 

 the availability of appropriate building blocks must be reckoned among 

 the requirements for growth. Such phenomena are now generally inter- 

 preted to mean that the cells lack the machinery for the synthesis of 

 essential compounds, which must consequently be supplied. It has fre- 

 quently been shown that the same organisms can also develop in an en- 

 vironment which contains, instead of the enzyme part itself, substances 

 that are chemically more or less closely related to it, and from which the 

 cellular constituent in question is elaborated. 



Corresponding observations have been made pertaining to the syn- 

 thesis of specific amino acids, as the epoch-making investigations of 

 Beadle and Tatum and their collaborators with "biochemical mutants" 

 have so amply demonstrated. Through studies of this kind it has been 

 possible to elucidate various steps in the biosynthesis of a number of 

 cell materials and to develop the thesis that each step is controlled by 

 a specific gene. It is impossible to do this field justice here; reference is 

 made to some recent reviews (37, 38). 



No student of the chemistry and physiology of growth is likely to 

 deny the importance of these advances in our knowledge of biosynthetic 

 mechanisms. Nevertheless the inclusion of this phase may appear as a 

 digression from my topic. What have such problems to do with the 

 kinetics of growth of microorganisms? 



