106 PLANT PHYSIOLOGY 



combustion into mechanical energy of motion. This comparison 

 however is not correct, since the Hving cell does not act like a 

 "heat engine, and the heat it liberates is not an intermediate but 

 a final stage of the intracellular transformations of energy. 



The transformations of energy connected with respiration in 

 the living cell are far from being understood in all their details, 

 just as the chemical processes involved in respiration are not 

 completely clear. A considerable advance in the analysis of 

 this question has been made through studies of oxidation-reduc- 

 tion potentials. 



Oxidation for a long time was considered as being the addition 

 of oxygen to the oxidizable molecule, and reduction as being the 

 removal of oxygen. As has already been seen, further study of 

 the reactions involved have led to considerable modifications of 

 this idea. The dye, methylene blue, for instance, may be easily 

 obtained by means of oxidation of its chromogen, a colorless 

 leuco compound, but this oxidation consists not in the addition 

 of oxygen to the leuco compound, but in the removal of two of 

 its hydrogen atoms, which form with oxygen a molecule of water. 

 Since it has been proved that such an oxidation with a removal 

 of hydrogen is very widespread, all losses of hydrogen should be 

 considered reactions involving oxidation. 



Further expansions of the concept of oxidation went so far 

 that this expression was applied not only to the transformation 

 of ferrous oxide into ferric oxide by the addition of oxygen, but 

 also to any transformation of ferrous into ferric forms, e.g., the 

 transformation of FeCU to FeCla. And as this transformation 

 is based on the change of valence of iron, the transformation of 

 the ferrous ion Fe"^ into a ferric ion Fe+++, the term ''oxidation'* 

 was applied to any acquisition of a positive charge by an ion; 

 from the viewpoint of the electron theory, this will be the case 

 of a loss of one or more electrons. According to this terminology, 

 reduction is the acquisition by an atom of one or more electrons. 



Thus the phenomenon of oxidation and reduction can be 

 combined with the phenomenon of gaining or losing electrons. 

 This concept made possible quantitative determinations of the 

 energy relations of these phenomena, based on the accompanying 

 changes in the electric charge of a system, where these processes 

 are taking place. If a platinum electrode is placed in an aqueous 

 solution of two salts of one and the same metal but of a different 



