FERMENTATION 157 



oxidation of about one-fifth of the total amount of lactic acid 

 which has disappeared. 



About the same time, Meyerhof * showed that when muscle 

 contracts glycogen disappears in an amount exactly equivalent 

 to the lactic acid produced, and that during the recovery 

 process the lactic acid disappears only in part by oxidation, 

 the remainder being resynthesized to glycogen. 



He found, moreover, that for every molecule of oxygen 

 absorbed, one and a half molecules of lactic acid disappeared ; 

 but from the equation 



C 3 H 6 o 3 + 30, = 3CO :! + 3 H 2 o 



one molecule of oxygen could only oxidize one-third of a mole- 

 cule of lactic acid, hence the ratio 



Lactic acid disappeared i£ 



Lactic acid oxidized £ 



4-5- 



Hill's experiments show that this ratio is approximately 5 ; 

 thus it appears that four-fifths of the lactic acid produced 

 are reconverted into glycogen at the expense of the energy 

 supplied by the oxidation of one-fifth, hence the anaerobic 

 process is reversed by the aerobic phase introduced by the 

 oxygen. Expressed in another way, when the oxidative pro- 

 cess keeps pace with the anaerobic stage the products of that 

 stage will not accumulate. Meyerhof f subsequently found 

 that much the same phenomena occur in yeast. This he was 

 only able to establish on realising the fact, ignored by earlier 

 workers, that some yeasts are more sensitive to the influence 

 of oxygen than others. Thus he established that ordinary 

 brewers' yeast, the so-called bottom fermentation yeast, the 

 type used by Pasteur in his experiments, oxidises only one 

 molecule of sugar for every 99 molecules of sugar which it 

 ferments ; in this type of yeast oxygen respiration is, therefore, 

 very low, a state of affairs which Meyerhof considers to be due 

 to long-continued selective cultivation in special conditions 

 with the result that the yeast has almost lost the habit of 



* See A. V. Hill and Meyerhof : " Ergeb. Physiol.," 1923, 22, 299. 

 f Meyerhof : " Biochem. Zeit," 1925, 162, 43 ; " J. Gen. Physiol.," 

 1927. 8, 531. 



