i 5 4 RESPIRATION 



2. Lactic Acid Fermentation. — Here the action of the hydro- 

 gen is immediate, the pyruvic acid being reduced to lactic 

 acid before either of the two other alternatives is possible — 



2CH3 . CO . COOH + 4 H = 2CH 3 . CHOH . COOH. 



Lactic acid 



3. Butyric Acid Fermentation. — This results from the slow 

 reducing action of the hydrogen : acetaldehyde is first formed— 



2CH3 . CO . COOH + 4 H = 2CH3CHO + 2C0 2 + 4 H. 

 The acetic aldehyde then undergoes an aldol condensation, 



2CH3CHO = CH 3 . CHOH . CH 2 . CHO 

 which, by molecular rearrangement, gives rise to butyric acid — 



CH 3 . CHOH . CH 2 . CHO = CH 3 . CH 2 . CH 2 . COOH. 



These three reactions can be expressed by the single equation — 



C 6 H 12 6 = CH 3 . CH 2 . CH 2 . COOH + 2CO s + 2H 2 . 



Reverting to the subject of alcoholic fermentation, the 

 views of Kostyschev are not shared by Neuberg who con- 

 siders that pyruvic aldehyde is first formed by the removal 

 of two molecules of water — 



C 6 H 12 6 -> 2CH 3 . CO . CHO + 2H 2 0. 



Assuming one molecule of pyruvic aldehyde to act in its 

 tautomeric form, the two molecules are supposed to react in 

 the presence of water according to the Cannizarro reaction — 



CH 3 CO CHO + O CH3CO COOH 



->• Pyruvic acid 



CH 2 = COH CHO + H 2 + H,0 CH 2 OH . CHOH CII 2 OH. 



Glycerol 



The pyruvic acid thus produced is decomposed by the enzyme 

 carboxylase into acetic aldehyde and carbon dioxide — 

 CH 3 CO COOH = CH 3 CHO + H 2 0. 



The acetic aldehyde then becomes reduced to ethyl alcohol 

 by acting as hydrogen acceptor for the hydrogen set free 

 from water when a second molecule of pyruvic aldehyde be- 

 comes oxidised — 



CH 3 . CO . CHO + O CH3CO . COOH 



— > Pyruvic acid 



CH 3 CHO + H 2 CH 3 CH 2 OH. 



Ethyl alcohol 



