ENERGY METABOLISM 421 



transformation. In the first place the molecule of sugar, 

 or any other carbohydrate, does not break down spontane- 

 ously at ordinary, comparatively low temperatures and it is 

 therefore difficult to liberate the energy locked up in it. For 

 this to occur, a very high energy barrier must be surmounted. 

 In the second place, if the molecule of sugar were broken 

 down or oxidised completely and suddenly, there Avould be 

 something like an explosion, which Avould be associated with 

 such heating of the protoplasm at the point of the occurrence 

 that its existence would be rendered impossible. In the 

 course of their evolution, therefore, organisms have elabor- 

 ated chemical mechanisms of energy metabolism in which 

 sugar is broken do^vn gradually, by stages, rather than sud- 

 denly. 



This gave the possibility, not only of surmounting the 

 barrier of the energy of activation of the separate reactions 

 at ordinary temperatures, but also of making rational use of 

 the energ}% which is then not liberated explosively, but step 

 by step in separate portions. 



The energy of organic compounds liberated in this way 

 can usually be accumulated in high-energy compounds which 

 can then be used, by means of specific mechanisms, for the 

 synthesis of proteins, for muscular contraction, etc. 



It must not, however, be supposed that energy exchange 

 takes place in the living cell as an isolated mechanism, 

 serving merely for the production of high-energy molecules. 

 In all processes of the biological destruction of organic sub- 

 stances (during fermentation, respiration, etc.) the straight- 

 forward task of storing energy is achieved, but the transfer 

 of electrons and hydrogen also takes place continually, as 

 well as the formation of those small fragments of the original 

 organic molecules which arise as intermediate breakdown 

 products and from which, in fact, the important components 

 of living material are directly synthesised. In this way organ- 

 isms are enabled to synthesise the tremendous variety of 

 extremely complicated substances which make up their 

 bodies, by the degiadation of a small number of non-specific 

 substrates. 



By virtue of all this, the energy metabolism of any organ- 

 ism consists of a long chain (or even many chains) of well 



