CHAPTER I 



INTRODUCTION 



Energetics of biological systems 



In recent years the attempts to analyse the energetics of 

 biological systems in terms of established thermodynamic 

 principles have naturally focused much attention on the 

 reactions in such systems which yield energy and those 

 which utilize energy [6]. When energy is supplied to or 

 liberated in a system, there are limitations regarding the 

 conversion of one form of energy into another (Second Law 

 of Thermodynamics). In other words, only part of the 

 energy content of any system is available for doing further 

 work, and this useful energy is termed /r^^ energy. Chemical 

 reactions in which there is an output of free energy are 

 described as exergonic and those in which there is an uptake 

 of free energy as endergonic. Reproduction, growth and the 

 maintenance of life are all endergonic processes and are 

 therefore intimately associated with mechanisms able to 

 supply them with energy. 



It is generally believed that energy becomes available in 

 biological systems as the direct or ultimate result of oxida- 

 tion reactions [8, lo]. The oxidation of one substance must 

 necessarily be accompanied by the reduction of another 

 and a biological oxido-reduction reaction involves the trans- 

 fer of hydrogen atoms or electrons [14]. Consequently the 

 substance which is oxidized is sometimes described as being 

 a hydrogen donor [H-donor], whilst the one being reduced 

 is termed the hydrogen acceptor [H-acceptor]. The com- 

 plete oxidation of any one substance proceeds by one or 

 more simple steps, each catalysed by the appropriate 

 enzyme, and in all known reactions the transfer of hydrogen 

 atoms or electrons to the ultimate H-acceptor is effected by 

 one or more intermediate carriers. In aerobic organisms, 

 molecular oxygen serv^es as the H-acceptor and, according 

 to the enzyme concerned, the end-product is water or 



