RESPIRATION OF PLANTS 103 



tchev, Embden, Meyerhof, and others) and is one of the most 

 important oxidation-reduction processes occurring in the Hving 

 organism. 



According to Wieland, it is the mechanism for sphtting off 

 hydrogen that is catalyzed, and he terms this ''catalyst activa- 

 tion." The removal of oxygen from the system in the form of 

 water is the result of this activation; for the activated hydrogen 

 is capable of reducing even molecular oxygen, hydrogen peroxide, 

 H2O2, being formed first and then water. The accumulation of 

 toxic peroxide in the organism of plants is inhibited by the 

 enzyme catalase always present in the cell. It decomposes 

 peroxide with the liberation of free oxygen. In the process of 

 respiration, this oxygen is again combined by the activated 

 hydrogen. 



According to Wieland's theory, the chief catalysts of respira- 

 tion are dehydrogenases, enzymes liberating hydrogen. The 

 most important among them are the aldehydrogenases, catalyti- 

 cally accelerating the destruction of aldehydes and playing an 

 important role in the process of fermentation; the peroxidases, 

 contributing to the oxidation of polyphenols, the respiratory 

 pigments of Palladin; and the hydrogen acceptors, participating 

 in the oxidation of intermediate products of fermentation. 



The chemist Schonbein (1860) was the first to formulate the 

 idea of the activation of oxygen. He noticed that many chem- 

 ical substances that by themselves do not oxidize in the air begin 

 to oxidize in the presence of some autooxidizable substances. To 

 explain this fact, Schonbein made the suggestion that oxygen is 

 first combined by these easily oxidizable substances and there- 

 upon is transformed into an easily activated form. 



Proceeding on the basis of these views, which were further 

 elaborated by Traube and Engler, Bach constructed his theory 

 of the action of oxidizing enzymes, or oxidases. This idea 

 Includes the concept of organic peroxides, compounds of the 

 general formula: 



The oxygen of these substances may be partly or wholly 

 liberated in the form of separate atoms with unsaturated affin- 

 ities and consequently capable of oxidizing substances that are 



