MECHANISM OF RESPIRATION 163 



is oxidized almost as soon as it is formed : but however this 

 may be, more information is necessary before an adequate 

 attempt can be made to correlate the catabolic processes of 

 these and like plants possessed of such plastic metabolic 

 methods. 



The work of Blackman and Parija on the respiration of 

 ripening apples has been mentioned on earlier pages. From 

 the results obtained, Blackman * has formulated a working 

 hypothesis of respiration. The phenomenon is a series of 

 phases, the ultimate products of one being the reactants of 

 the next, but the fate of the reactants of the last phase, being 

 controlled by the supply of oxygen, is diverse. 



A.— The first phase is essentially that of hydrolysis, the 

 splitting of various carbohydrate reserves to form free hexose 

 which is not directly respired. 



B. — The hexose is activated to a " heterohexose " pos- 

 sessed of a less stable type of internal ring structure. 



C. — The heterohexose undergoes disintegration, glycolysis, 

 to various intermediate products, of which methyl glyoxal, 

 lactic acid, pyruvic acid and acetaldehyde are examples, 

 which form the raw material for the last phase D. This gly- 

 colysis proceeds at a greater rate in air than in nitrogen and 

 is still further accelerated by higher oxygen pressures. This 

 acceleration is not due to oxidation but to the hastening of 

 the rate of production of the' substrate for glycolysis. 



D. — The fate of the intermediate products of the phase C 

 is dependent on the absence or presence of oxygen. In an- 

 aerobic conditions alcohol and carbon dioxide are produced ; 

 in aerobic conditions water and carbon dioxide are the final 

 waste products, together with an oxidative anabolite. 



The early processes are reversible reactions; hexoses by 

 condensation can give rise to the original carbohydrate reserve, 

 and the heterohexoses can pass back to hexoses. The other 

 phases are considered to be irreversible. 



The oxidative anabolism in the last phase is a deduction to 

 explain the differences which obtain in aerobic and anaerobic 



* Blackman : " Proc. Roy. Soc," B, 1928, 103, 491. 



II* 



