542 ANAEROBIC RESPIRATION 



tion of it, which has been strongly supported by Kostychev (1927), is now 

 rather generally regarded as at least an excellent working hypothesis of the 

 inter-relationship of the two types of respiration occurring in the cells of higher 

 plants. This theory is illustrated schematically in the following diagram: 



2C2H5OH+ 2 CO 2 + 25 kg.-cal. 



f 



zymase 



Intermediate products In absence of O2 



CgHiaOe zymase^ of anaerobic respiration In presence of O2 



Oxidizing-reducing 

 +6 O2 \enzymes 



^6 CO2+6H2O+ 673 kg.-cal. 



As this diagram indicates certain enzymes of the zymase complex are first 

 supposed to convert the sugar into labile intermediate products, this anaerobic 

 stage being the first step in both aerobic and anaerobic respiration. After 

 this step the course of the reaction is supposed to depend upon whether or not 

 oxygen is available. If not, the reaction proceeds anaerobically, alcohol and 

 carbon dioxide being produced from the intermediate products, probably as a 

 result of the action of other enzymes of the zymase complex. If oxygen is 

 available, then complete oxidation of the intermediate products to carbon 

 dioxide and water results under the influence of oxidizing-reducing enzymes. 



Among the reasons for believing in the essential validity of this theory are : 



1. The fact that anaerobic respiration is apparently of universal occurrence 

 in higher plants when deprived of oxygen. 



2. The apparently invariable presence of the enzyme zymase in the cells 

 of higher plants. 



3. Some of the same intermediate products, as for example acetaldehyde, 

 which are found in the anaerobic respiration of hexose sugars, have also been 

 detected in plants during aerobic respiration. Gustafson (1934), for example, 

 has shown that acetaldehyde apparently is always present in respiring tomato 

 fruits. 



4. When seedlings are "fed" fermented sugar solutions a great increase in 

 the rate of respiration can be observed, indicating that certain intermediate 

 products of alcoholic fermentation (hence by inference of anaerobic respira- 

 tion) can serve as the respiratory substrate. ' 



5. When oxygen is again allowed access to plant tissues which have been 

 deprived of it for some time, a temporary increase in the rate of aerobic res- 

 piration can usually be observed. This appears to indicate the accumulation 

 during anaerobic respiration of oxidizable substances in the cells, and that this 



