208 ANAEROBIC METABOLISM 



aerobic as well as the anaerobic degradation of glucose is 

 enhanced by carbon dioxide but only the aerobic degrada- 

 tion of glycerol. The exact nature of the carbon dioxide 

 assimilation is not yet known ; Searle and Keiner (1940) 

 assume that it is related to oxidation-reduction processes 

 and suggest (1941) that both the anaerobic carbon diox- 

 ide assimilation and the aerobic stimulation of the metab- 

 olism by carbon dioxide involve the formation of glucose 

 carbonate and glycerol carbonate. 



Under aerobic conditions the initial steps in that se- 

 ries of reactions are the same as under anaerobic condi- 

 tions. But under aerobic conditions, the acetaldehyde 

 is transformed into formic acid, carbon dioxide and water. 

 The greater part of the formic acid is then completely oxi- 

 dized to carbon dioxide and water. Another direct oxy- 

 genation may transform some of the acetaldehyde into 

 acetic acid. The identified end products of aerobic sugar 

 decomposition are therefore formic, acetic and succinic 

 acid, ethyl alcohol and carbon dioxide. The R.Q. of these 

 processes should theoretically be 0.8, but actually it w^as 

 found to be 0.98. This indicates that there are still some 

 gaps in our knowledge. 



The fermentative processes of the protozoa inhabiting 

 the intestine of Zootermopsis nevadensm and Zooter- 

 mopsis angusticollis w^ere studied primarily by Hungate 

 (1939, 1943a). He showed that these protozoa decompose 

 cellulose and he was able to identify about three-fourths 

 of the end products of this decomposition. Among these 

 were carbon dioxide, hydrogen and organic asids, the 

 most important of the latter being acetic acid. In one 

 case at least some lactic acid was also formed. It is to 

 be noted that two types of fermentation occurred in which 

 the various end products appeared in different propor- 

 tions but, so far, these fermentations have not been 

 found to be attributable to specific protozoan species of 

 which large numbers would occur in this habitat. Hun- 



