148 PAUL WEISS 



can occur only under specified conditions which will vary for different 

 compounds. The various chemical systems of the cell thus form "sym- 

 biotic" interdependences, and the integrity of the cellular system as a 

 whole is contingent on their coexistence and cooperation. For each 

 particular chemical system we can thus postulate a definite set of "exis- 

 tential and operational prerequisites," that is, conditions indispensable 

 for the synthesis, preservation, and operation of specific organized 

 products. The character of a given animal population is determined by 

 the physical environment (climate, physiographic factors, etc.) and by 

 the organic environment (food, competition among species, biocoenosis, 

 etc.) in that particular area. Similarly, the character of the molecular 

 population occupying a given site in a cell will depend on the physical 

 conditions (electric potentials, surface tensions, etc.) and the chemical 

 interactions among the various molecular species in that locality. 



The ecological simile is primarily a convenient device to describe the 

 behavior of the molecular populations of cells. It permits us, for instance, 

 to answer in principle the question of how the morphological segrega- 

 tion and orderly distribution of different chemical systems in the cell 

 is brought about, preserved, and restored after disturbance, despite the 

 absence of static internal frameworks. If each complex biochemical 

 system requires highly specific conditions for its formation and opera- 

 tion, then obviously the probability of finding that kind of system will 

 be high at sites where those conditions are satisfied, and low at others. 

 And after commingling, the content will again sort itself out according 

 to the frame of marginal conditions, and if the latter have remained 

 undisturbed, the re-sorting will likewise follow the old order. We will 

 thus find preferential sites for the synthesis of certain compounds, 

 namely, sites where the necessary ingredients, templates, energy sources, 

 and physical factors for that particular synthesis coexist. There must 

 also be preferential sites for the settlement of the finished products ; for 

 as synthesis continues, the accumulating products are driven further and 

 further from their production centers and thus brought under conditions 

 not necessarily compatible with their continued existence. I have sug- 

 gested on a previous occasion (54) that such preferential settlement 

 might be a matter of the configuration of the molecule, as a stable layer 

 of molecules with specifically shaped ends firmly adsorbed to an interface 

 could readily trap roaming molecules of complementarily fitting con- 

 figuration.' 



3 This hypothesis leans on the immunochemical concepts of Pauling (32) . 



