TEMPORAL ORGANIZATION IN CELLS 



The discovery of these molecular control mechanisms is of unparalleled 

 importance for the understanding of cell behaviour, and it is upon this founda- 

 tion that any theory of cellular organization must be constructed. However, 

 the purely qualitative description of what appear to be the fundamental control 

 mechanisms of cells does not tell us about the dynamic properties of the system, 

 the kinetics of macromolecular synthesis and control. It will be necessary to 

 construct such a dynamics on the basis of what we know about the general 

 kinetics of molecular activities in cells, and also on the basis of the dynamic 

 behaviour of feed-back control devices such as those commonly used in 

 engineering. This procedure will of necessity be rather approximate, but it is 

 just at this point that a second consideration encourages the investigation of 

 even crude kinetic models of cellular activity. 



If the dynamic equations describing the kinetics of biochemical control 

 systems in cells can be used as the basis for constructing a statistical mechanics 

 of cellular control processes, then the macroscopic or "thermodynamic" 

 properties which will emerge from the statistical mechanics will describe very 

 general features of cell behaviour. Even if the equations are only a rough 

 approximation to the actual dynamics of molecular activity in cells, they may 

 nevertheless give important information about "thermodynamic" properties 

 of the system, for it is the nature of a statistical mechanics that many of the 

 microscopic details are smoothed out, as it were, and only the fundamental 

 dynamic properties are retained. Therefore, in spite of the incompleteness of 

 present knowledge about the molecular organization of cells, we may never- 

 theless get some idea of the macroscopic quantities which are relevant to a 

 general description of cell behaviour, and some suggestion of experimental 

 procedures for controlling and observing these macroscopic quantities. 

 However, there must obviously be a basic qualitative similarity between the 

 essential dynamic features of the kinetic model of cellular control mechanisms 

 and the real system, in order that a statistical mechanics and thermodynamics 

 produce meaningful and useful results for the study of integrated cell behaviour. 

 The experimental predictions which arise from the theory then constitute a test 

 of this basic similarity, and discrepancies between prediction and observation 

 will indicate either that the model is a complete failure or that it is essentially 

 valid but requires modification. The encouraging feature of a programme 

 which can utilize the procedures of statistical mechanics is the possibility of 

 testing general properties of the kinetic model rather than specific or micro- 

 scopic features. In Chapter 4 we will derive a set of equations which describe 

 a certain type of dynamic control system consistent with our present know- 

 ledge of molecular regulation, and in Chapter 5 we will construct a statistical 

 mechanics on the basis of these equations. Some "thermodynamic" proper- 

 ties of the system are then deduced, and in Chapter 8 we give suggestions of 

 how these may be tested against the behaviour of real cells. 



This brings us finally to the third aspect of the present situation in biology 

 which encourages the investigation of thermodynamic-like theories for the 

 description and analysis of cell behaviour. There has emerged recently a field 

 of biological study in which it is possible to carry out precise observations and 



