324 ORGANISATION IN SPACE AND TIME 



from the group of open systems, to which living organisms 

 belong. 



In enclosed systems the only things which can react 

 chemically with one another are substances which are present 

 in the system. The constancy of the properties of the system 

 over a period is characterised by a state of equilibrium in 

 which the rate of a reaction in one direction is the same as 

 the rate of the same reaction in the opposite direction. The 

 thermodynamic criterion for this equilibrium is the presence 

 of the minimal amount of free energy and the maximal 

 amount of entropy in the system (in other words the attain- 

 ment of the most probable state of the system). Processes 

 occurring spontaneously within an enclosed system cannot 

 cause it to reach a less probable state, that is to say, they can 

 only maintain the entropy at its existing level or increase 

 it, according to whether the processes in question are revers- 

 ible or irreversible. So long as the entropy of a system is 

 increasing, equilibrium has not been reached and, conversely, 

 when equilibrium is set up, the rate of increase of entropy 

 falls to zero. 



In contrast to this, in open systems there is a continual 

 accession of substances from the external medium into the 

 system (from which it is separated in some way) and also a 

 discharge of chemical substances, which arise within the 

 system, back into the external medium. The constancy in 

 time of the properties of such an open system is, therefore, 

 not characterised by thermodynamic equilibrium (as is the 

 case in enclosed systems) but by the setting up of a stable 

 condition, the constancy of which is maintained by the rate 

 at which chemical reactions proceed in one direction and 

 bv the diffusion of substances within the system. 



Stationary processes may, of course, occur in closed systems 

 though not in isolated ones,"^ for example, the transfer of 

 heat. The stationary state in which we are interested is that 

 involving chemical reactions and this is peculiar to open 

 systems. We shall therefore direct our attention to these. 



Thermodynamic equilibrium and the stationary state 

 resemble one another in that, in both cases, the constancy 

 of the properties of the system is maintained. The essential 

 difference between them is that in thermodynamic equi- 



