MECHANISM OF AEROBIC RESPIRATION 545 



oxidations and reductions. In the following brief discussion it will be possible 

 for us to touch on only a few of the more important suggestions which have 

 been advanced regarding the mechanism of the aerobic phase of plant 

 respiration. 



Palladin (1909) proposed a theory of aerobic respiration which continues 

 to merit consideration. This theory will purposely be discussed in a somewhat 

 more generalized form than that first proposed by Palladin, and can be 

 represented by the following equations: 



Dehydrogenase - „,-- , . tt 



CeHisOe + 6 H2O + 12 A > 6 CO2 + 12 AH2 



Intermediate product? Hydrogen Reduced 



of anaerobic respiration acceptor acceptor 



„ Oxidase . tt ^>> 



12AH2 + 6O2 > 12 A +12H2O 



Reduced Hydrogen 



acceptor acceptor 



In the first stage of the process the intermediate products of anaerobic 

 respiration (acetaldehyde, etc.) are supposed to be oxidized by the active oxy- 

 gen remaining after the dehydrogenation of water, the hydrogen released 

 combining with a hydrogen acceptor (A). Since the exact chemical nature 

 of the intermediate products of anaerobic respiration is uncertain, represen- 

 tation of this reaction is simplified by indicating them by the formula 

 C6H12O6. Such a reaction is probably catalyzed by a dehydrogenase. The 

 oxygen utilized does not come from the atmosphere, but from the water 

 molecules. 



In the second stage of the process the reduced hydrogen acceptor (AH2) 

 is supposed to be oxidized to its original condition. This reaction is presum- 

 ably catalyzed by oxidases which as a system possess the capacity of activating 

 atmospheric oxygen. According to this view atmospheric oxygen is consumed, 

 not in direct oxidation of the substrate, but in the oxidative regeneration of 

 the hydrogen acceptors. 



In Palladin's own formulation of this theory the role of hydrogen accep- 

 tors was ascribed to certain types of pigments found in plants. These he 

 termed respiratory pigments. In the first stage of the process the respiratory 

 pigment is supposed to be reduced to a colorless form or chromogen (the 

 behavior of methylene blue, previously described, is analogous), while in the 

 second stage this reduced pigment or chromogen is oxidized back to the pig- 

 ment. Palladin's theory, in the restricted form in which he proposed it, has 

 often been called the chromogen theory of respiration. 



Pigments which can be shown experimentally to react in the manner pos- 

 tulated by Palladin have actually been isolated from the tissues of some plants, 



