546 ANAEROBIC RESPIRATION 



and doubtless occur in many others. Such pigments invariably seem to be 

 phenol compounds which is in accord with the fact that oxidases apparently 

 can oxidize only substances containing a phenol grouping. 



However, it is not certain that respiratory chromogens occur in all plant 

 tissues, and it is well known that all plant tissues do not contain oxidases. 

 That this theory, in its original restricted form, is universally applicable to 

 plants is therefore doubtful. The possibility is by no means precluded, how- 

 ever, that other substances besides respiratory pigments may serve as hydrogen 

 acceptors in the respiratory process. Indeed, the results of recent investigations 

 indicate that this is very probable. The widespread occurrence of cytochrome 

 and glutathione in plant and animal tissues has already been mentioned. Both 

 of these substances can serve as hydrogen acceptors. After combining with 

 hydrogen they can be oxidized back to their original state by the loss of hydro- 

 gen ; i.e. can also serve as hydrogen donators. Such compounds might therefore 

 act as hydrogen acceptors in a respiratory mechanism otherwise essentially 

 similar to that proposed by Palladin. Other compounds possessing similar 

 properties may also occur in living cells. Some such substances may be able 

 to accomplish oxidations and reductions without the intermediation of en- 

 zymes, but the evidence upon this point is not 3Tt clear. 



While theories of the mechanics of aerobic respiration of this general tj'pe 

 have by no means been fully substantiated, they do serve as provocative hypoth- 

 eses and are in general accord with present factual knowledge. 



Comprehensive investigations have been carried out in recent years by 

 Blackman and his associates on aerobic and anaerobic respiration in apple fruits. 

 As a result of these studies certain hypotheses regarding the mechanism of 

 respiration have been proposed (1928). In general the conclusions drawn 

 from the results of these investigations agree with the theories of respiration 

 which have just been described. Two additional points are stressed by 

 Blackman, however. In the first place oxygen appears to be required, not 

 only in the oxidation of the intermediate products of anaerobic respiration, 

 but also seems to have an important effect in the "activation" of the hexose 

 molecules. In other words, with increase in the availability of oxygen the rate 

 of conversion of hexoses into an active form (presumably gamma-glucose or 

 gamma-fructose) is increased. The second additional point brought out by 

 Blackman's investigations is that not all of the carbon in the compounds 

 formed as a result of the action of the zymase complex {i.e. the intermediate 

 products of anaerobic respiration) is oxidized to carbon dioxide. Since, ac- 

 cording to Blackman, about three-fourths of the carbon in these compounds is 

 not oxidized it is obvious that a large proportion of the products of zymase 

 activity must remain in the cells. According to Blackman these products are 



