544 ANAEROBIC RESPIRATION 



4. The pyruvic acid is then split immediately into acetaldehyde and carbon 

 dioxide, under the influence of carboxylase, another enzyme of the zymase 

 complex : 



CH3 • CO ■ COOH ^^^^^^ CH3 • CHO + CO2 



Pyruvic acid Acetaldehyde 



5. Up to this point the process of respiration is supposed to proceed 

 along the same course, whether it occurs under aerobic or anaerobic condi- 

 tions. Under anaerobic conditions the next step is supposed to be a second 

 Cannizzaro reaction in which a molecule of the methyl glyoxal produced in 

 stage 2 reacts with a molecule of the acetaldehyde produced in stage 4 as 

 follows : 



CH3COCHO CHs-CO-COOH 



Methyl glyoxal Q Pyruvic acid 



+ + II -^ + 



CHb-CHO ^^ CH3CH2OH 



Acetaldehyde Ethyl alcohol 



The resulting pyruvic acid is supposed to be acted on by carboxylase as in 

 stage 4, but the alcohol is not subject to further chemical change, and becomes, 

 together with the carbon dioxide evolved in stage 4, one of the final end 

 products of the reaction. 



In another recently proposed theory of alcoholic fermentation phosphate 

 esters of the carbohydrates are supposed to be intermediate compounds in the 

 reaction rather than methyl glyoxal (Aleyerhof and Kiessling, 1935)- It is 

 supposed that hexose diphosphates are first formed, that these are converted 

 into a compound composed of a triose combined with phosphoric acid which 

 passes through several intermediate stages, finally resulting in the production of 

 pyruvic acid. Acetaldehyde is then supposed to be formed from pyruvic acid 

 by loss of carbon dioxide, and ethyl alcohol to be formed from the acetalde- 

 hyde. 



Theories of the Mechanism of Aerobic Respiration. — Up to and in- 

 cluding the stage in which "intermediate products" such as acetaldehyde are 

 formed it is generally supposed that the progression of stages in both aerobic 

 and anaerobic respiration is identical. The problem yet remaining before us 

 is that of the steps by which the intermediate products of anaerobic respiration 

 are converted into carbon dioxide and water in the process of aerobic respira- 

 tion. 



Many extensive investigations of the mechanism of biological oxidations 

 have been undertaken upon both plant and animal tissues, and a number of 

 rather diverse opinions have been entertained regarding the nature of cell 



