90 



OXIDATION-REDUCTION POTENTIALS 



PASTEUR EFFECT 



Pasteur (1861) observed that the amount of sugar fermented by yeast was dimin- 

 ished in the presence of oxygen. In parenthesis, Barron (1943) light-heartedly com- 

 ments that if Pasteur could have foreseen the misuses and misinterpretations of his 

 observation he would never have mentioned the subject. 



The utilisation of sugar in anaerobic fermentation is a relatively inefficient 

 process and for the production of the same weight of yeast cells some ten times as 

 much sugar is consumed anaerobically as in the presence of an abundant oxygen 

 supply. A crude analogy here might be that of a motor car being driven with the 

 air choke in action. For starting on a cold morning the use of the choke in cutting 

 down the air supply and giving a " rich mixture " may accelerate starting ; but if the 

 air supply is cut down for normal running the petrol consumption will be grossly 

 excessive for the power developed. 



The relative inefficiency of anaerobic fermentation follows from free energy 

 considerations. In the complete oxidation of glucose to carbon dioxide 686,000 

 calories of free energy are liberated, but as indicated previously the theoretical 

 maximum of free energy liberated during anaerobic glycolysis of glucose to lactic 

 acid can only be 58,000 calories. Aerobic oxidation of sugar, therefore, yields nearly 

 12 times as much free energy for the performance of useful work compared with 

 anaerobic glycolysis. As mentioned previously this free energy is stored by organisms 

 in the form of energy-rich phosphate bonds (r-^ph). Since some 3 or 4 r-^ph are made 

 available in glycolysis it would appear that 36 to 48 r-^ph appear after aerobic oxida- 

 tion of each molecule of glucose. Each r-^ph bond accounts for nearly 12,000 calories 

 of free energy. The mechanism by which these appear during the aerobic tricarboxyUc 

 acid cycle has not yet been detailed. 



The mechanism by which aerobiosis inhibits fermentation has been the subject 

 of much work and even more discussion. 



The Meyerhof quotient : — 



anaerobic fermentation — - aerobic fermentation -^ respiration 

 gives the number of molecules of glucose which are inhibited from being fermented 

 by every molecule of oxygen absorbed. Some values (after Lipmann) are given in 

 Table 20. 



TABLE 20 

 Meyerhof Quotients of Microorganisms 



