52 A SYMPOSIUM ON RESPIRATORY ENZYMES 



to take the theoretical caloric value of 54 and 686 kg.-cal., respec- 

 tively, as a basis for comparing their efficiencies, and to conclude 

 that only one-twelfth to one-ninth of the total possible energy is 

 made available to the cell by the anaerobic fermentation of the 

 glucose molecule. 



From this calculation the superior economy of respiratory me- 

 tabolism becomes evident. To draw the same amount of energy 

 from fermentation as from respiration the cell must use from nine 

 to twelve times as much substrate. In reahty the anaerobic energy 

 is rarely equal to the aerobic. As a rule a fully developed facultative 

 anaerobe uses anaerobically only four to eight times as much sub- 

 strate as aerobically, thus reaching on the average half the energy 

 level of the aerobic state. From these considerations the economical 

 and regulatory aspect of the Pasteur effect becomes evident. 

 Through its operation the voluminous fermentative metabolism is 

 allowed to proceed only in anaerobiosis, as is indicated in the fol- 

 lowing scheme, which represents the increase in glucose utilization 

 following change from aerobic to anaerobic conditions: 



OXYGEN NITROGEN 



(Qo, = 6; Q^^ = 0) (Q^^=10) 



1. Glucose + 6 Oj -> 6 CO2 + 6 H2O 1. Glucose -> 2 Lactate 



2. Glucose — > 2 Lactate 



3. Glucose — > 2 Lactate 



4. Glucose — > 2 Lactate 



5. Glucose — > 2 Lactate 



Some examples of "ideal" facultative anaerobic cells are given in 

 Table 1. From the Q values the corresponding glucose consumption 

 and the caloric yields are calculated. One cubic millimeter of respira- 

 tion oxygen corresponds to the utilization of 1.34 micrograms of 

 glucose and the yield of 5.2 X 10^^ cal.; one cubic millimeter of 

 fermentation carbon dioxide corresponds to the utilization of 4.03 

 micrograms of glucose and the yield of 1.2 X 10^^ cal. 



In the cases cited the large fermentative metabolism disappears 

 completely in aerobiosis. In the experiment with the fish retina 

 almost three-fourths of the aerobic energy is made available through 

 anaerobic metabolism and correspondingly a three- to sevenfold an- 

 aerobic increase of glucose consumption occurs. 



The Metabolic Structure of Cells 



In the middle of the metabolic type scale are placed the organ- 

 isms that alternate between anaerobic and aerobic metabolism of 



