THERMODYNAMICS AND KINETICS 323 



The thermodynamics and 

 kinetics of open systems. 



The mechanistic view of the organisation of living bodies 

 which prevailed among biologists until recently, namely that 

 they were like machines made up of immutable steel com- 

 ponents, made such a concept of organisms as open systems 

 very difficult to accept. However, the use of marked atoms 

 in biochemical and physiological investigations^® has shown 

 beyond doubt that almost all the substances of the living 

 body, its proteins, nucleic acids, lipids, etc., are completely 

 renewed in the course of a short space of time ; that the 

 material substrate of life is constantly being exchanged with 

 the surrounding medium, it is continually being broken 

 down and synthesised again fi'om substances derived from 

 the external world. This provided a complete vindication 

 of Michurin's principle of the unity of the organism and the 

 environment ; the contention that a living thing cannot 

 be considered in isolation from its environment, without 

 reference to this unity.®'^ 



On the other hand, the contemporary wide adoption in 

 industrial practice of technological methods based on con- 

 tinuous irreversible processes has led many physicists and 

 chemists to undertake a complete revision of the theory of 

 open systems, which has introduced many new concepts into 

 the classical thermodynamic and kinetic theories, which are 

 mainly based on the kinetics and equilibrium of reactions 

 in completely isolated systems. 



In his very interesting book. An introduction to the 

 thermodynamics of irreversible processes, I. Prigogine®* 

 divides all limited systems into three fundamental classes : 

 (i) open, (2) closed and (3) isolated systems. The first group 

 comprises systems in which there is a constant exchange of 

 both matter and energy between them and their surround- 

 ings. In closed systems the exchange is only of energy, the 

 exchange of matter being absent. Finally, the third group 

 comprises systems which are completely isolated from their 

 surroundings and do not exchange either matter or energy 

 with them. The latter two groups may be combined under 

 the general term ' enclosed systems ' to distinguish them 



