62 RESPIRATION 



then may be oxidized to methylglyoxal, to be in turn transformed 

 sequentially into pyruvic acid, acetaldehyde, and, as a final prod- 

 uct, alcohol. The decarboxylation of pyruvic acid yields the 

 carbon dioxide evolved in the process. 



THE RESPIRATORY RATIO 



As is well known and has been stated previously, the complete 

 respiration of hexose yields a respiratory ratio of unity. Fungi, 

 however, respire not only hexoses but also various fats and organic 

 acids. When such substances are oxidized in the respiratory 

 process, it may be anticipated that the ratio of 2 consumed to 

 C0 2 released will differ from that shown by the respiration of 

 hexoses. The anaerobic respiration of oxalic acid, for example, 

 should yield a ratio of 4, as is indicated by the reaction 

 2(COOH) + 2 -> 4CO, + 2H 2 + 60.2 Cal. Again, it should 

 be anticipated that the ratio will be small if substances poor in 

 oxygen are respired completely, as appears from the reaction 

 involving the fat tripalmitin: 



C 5 iH 98 6 + 72.50 2 -> 51C0 2 + 49H 2 + 7590 Cal 



In this case the ratio is 5lC0 2 /72.50 2 , or 0.7. 



Richter's experiments with fermentation by yeast (1902) show 

 that factors other than the character of the substrate enter into 

 the problem of the respiratory ratio. He grew the organism 

 in large, flat-bottomed, hermetically sealed flasks containing 50 ml 

 of nutrient salt solution, consisting of K 2 HP0 4 , xMgS0 4 , and a 

 trace of Fe. To this solution he added varying amounts of sucrose 

 and peptone. In those to which he added 0.15 gram sucrose and 

 0.25 gram peptone, the C0 2 /0 2 ratio after 24 hours was 4.26; 

 after 48 hours, 2.25. In those to which he added 0.3 gram sucrose 

 and 0.5 gram peptone, the C0 2 /0 2 ratio after 24 hours was 8.32; 

 after 48 hours, 6.16. In those to which he added 0.75 gram sucrose 

 and 1.25 grams peptone, the CCX, 2 ratio after 24 hours was 

 11.16; after 48 hours, 27.46. From these experiments he concluded 

 that yeast utilizes sugar in preference to peptone as a source of 

 energy, but that the concentration of food in the substrate be- 

 comes an important factor in modifying both the respiratory 

 ratio and the rate of respiration. 



