K.— BOTANY 219 



If we cannot answer this question we can, at any rate, attempt an 

 examination of the functions of respiration of which we feel more certain. 

 The most universal of these, as we have seen, is the provision of energy 

 for the building up of materials of higher energy content. A problem 

 which awaits solution here is the mechanism by which the energy released 

 in the oxidation of the substrate is transferred to the actions bringing 

 about the synthesis of proteins and other complex plant constituents. 

 The solution of this problem no doubt involves that of what is generally 

 known as the mechanism of respiration, that is, the stages in the process, 

 the enzymes involved, the conditions of the process : in fact, the general 

 course of the breakdown of substrate into carbon dioxide and water. The 

 assumption is usually made that the breakdown of sugar follows the same 

 course in its earlier stages as in its fermentation by yeast, in which, accord- 

 ing to the theories of Neuberg and of Embden and Meyerhof, pyruvic 

 acid, CH3CO.COOH, is an intermediate product, and in which enzymes 

 of the zymase complex play a leading part. Both in presence and absence 

 of oxygen the course of the breakdown is supposed to be the same up to 

 the splitting of pyruvic acid under the action of the enzyme carboxylase 

 to acetaldehyde and carbon dioxide. The carbon dioxide evolved in 

 this action accounts for the whole of this gas evolved in the absence of 

 oxygen, and for one-third of the carbon in the sugar broken down, the 

 other two-thirds, contained in the acetaldehyde, finally appearing as 

 ethyl alcohol. If the rate of sugar breakdown, or glycolysis, remains the 

 same in both presence and absence of oxygen, then the ratio of anaerobic 

 to aerobic respiration depends on the fate of the acetaldehyde in air. If 

 none of this appears as carbon dioxide the ratio is unity, if all the carbon 

 contained in it appears as carbon dioxide the ratio is 1:3, and it must 

 be supposed that if the ratio exceeds this value some of the acetaldehyde 

 is built either into some fresh product or back into the system. Indeed, 

 this was realised as long ago as 1880 by Wortmann, who actually found 

 with seedlings of Vicia faba that the rate of carbon dioxide evolution was 

 the same in presence and absence of oxygen. To explain this finding 

 he put forward a theory that in air the alcohol produced in the first stage 

 of the process is converted back to sugar, so that the whole of the respira- 

 tory process can be summarised by two equations which he wrote as 

 follows : 



I. sCCeHi^Oe) = (^{c.,Yi,on) + eco^. 



2. 6(C2H50H) + 12 O = 2(C6Hi206) + 6H2O. 

 The second equation he also wrote : 





3. 6(C2H50H) + 12 O - 2 I C2H4O2 I + 6H2O. 



2(CeH,20e) 



In short, he accounted for the fact that the same amount of carbon dioxide 

 was released under anaerobic conditions as under aerobic conditions, by 

 supposing that two-thirds of the sugar broken down was re-synthesised 

 in presence of oxygen into sugar. 



