MECHANISM OF RESPIRATION 167 



which is known to be attacked by carboxylase giving rise to 

 pyruvic acid — ■ 



CH 2 . COOH CH 3 



I -> CO, + I 



CO . COOH CO . COOH. 



The resulting pyruvic acid can then be further converted 

 into acetic aldehyde which is itself broken down by a repetition 

 of the cycle of changes outlined above. 



Although the scheme appears to lead back to the original 

 material, it will be noted that two molecules of acetaldehyde 

 were involved at the beginning, but only one remains at the 

 end of the cycle. 



It will be noticed that many of these oxidative processes 

 are effected by the removal of hydrogen ; but since in the 

 respiration of the green plant hydrogen, as such, is never set 

 free, the hydrogen must in some way be fixed ; the current 

 view is that it is oxidized to water with the intervention of 

 a respiratory pigment, glutathione or some other mechanism. 

 Palladin was the first to visualize that atmospheric oxygen 

 was concerned only with the oxidation of the hydrogen of the 

 respirable material to water, a view supported by Thunberg 

 who regards hydrogen as the essential fuel of the living cell. 



The above considerations in the main are concerned with 

 sugar as the respirable substance ; but, as has already been 

 mentioned, fats and proteins are also available to meet the 

 energy requirements of plants, concerning which there is less 

 precise information. Oparin * has examined chlorogenic acid, 

 C 32 H 38 19 , which he has found to occur in over a hundred 

 different plants, and finds it to be a substance which readily 

 oxidizes in the air, losing 4 atoms of hydrogen and forming 

 a green pigment. The latter can act as a hydrogen acceptor 

 and so act as an oxidizing agent. The calcium salt of the 

 fully oxidized acid is represented by the formula CaC3 2 H 32 19 

 + 2H 2 0, whilst the salt of the unoxidized acid has the for- 

 mula CaC 32 H 36 19 + 2H 2 0. Chlorogenic acid is more par- 

 ticularly active in the oxidation of natural amino acids, 



* Oparin : " Biochem. Zeitsch.," 1921, 124, 90. Gorter : " Annalen d. 

 Chemie," 1908, 359, 217. 



