CHAPTER I 



GENERAL CONSIDERATION OF OXIDATION-REDUCTION 



PROCESSES 



Oxidation-reduction systems play so intimate and so essential a part in living 

 organisms that life itself might be defined as a continuous oxidation-reduction 

 reaction. It is not surprising, therefore, that theoretical speculations and experi- 

 mental studies on oxidation and reduction processes in animals and plants have been 

 actively pursued since the isolation of oxygen over 150 years ago. The dependence 

 of animal life on the maintenance of an adequate oxygen supply has, rather naturally, 

 dominated the outlook and led to the assumption that oxygen itself is an essential 

 reactant in all biological processes, a view which was vigorously criticised by Pasteur. 



The attention devoted to oxygen gas has tended to obscure the mechanism of 

 biological processes, many of which are not dependent upon the direct participation 

 of oxygen. Biological systems cannot be expected to yield to study unless there is 

 available a means of measuring their oxidising or reducing properties and of describing 

 in quantitative terms the stage of oxidation or of reduction reached in an oxidation 

 reduction reaction. Other methods of study than that of gas analysis and, above 

 all, a different aspect are necessary in order to proceed to even the faintest under- 

 standing of the fundamental features of biological processes. 



Organisms are constantly faced with the problem of obtaining the energy 

 necessary for their growth and existence and the problem is solved in different 

 ways by different organisms. Plants, for example, are able to utilise the energy 

 obtained from sunlight to effect reductive syntheses such as the classical one of 

 glucose from carbon dioxide and water, 



6 CO2 + 6 H2O = CgHiaOe + 6 Og 



Many other organisms reverse this process, oxidising glucose to carbon dioxide 

 and water, thereby deriving energy essential to themselves. Animals and many 

 bacteria belong to this latter class. The crude result, as represented by the 

 equation above is, however, of little value in studying the essential mechanism of 

 biological processes. The comjilete degradation of glucose to carbon dioxide and 

 water takes place by a chain of reactions, many of which are unknown or imperfectly 

 understood. We are particularly interested in those steps in the series of reactions 

 which are reversible and enable the living cell to store energy which may be utilised 

 as required. Such a reaction is readily reversed when the energy liberated in the 

 forward reaction is supplied to the system. 



Michaelis (1933) presents analogies between the chain of reactions in the degrada- 

 tion of glucose and an electrical supply circuit. In the circuit most of the current 

 is used for heating lamp filaments, etc., but an accumulator may be charged in the 

 circuit. This provides a reversible mechanism for storing energy. The accumulator 

 may be used, for example, to drive an electric motor or to obtain other forms of energy 

 and may be recharged by the main circuit when required. In an analogous manner, 

 a thermodynamically reversible reaction may be used by living cells to store energy 

 which is utilised when required — those reactions in the degradation of glucose 

 which are irreversible cannot be used for this purpose. 



