THE CHEMISTRY OF PHOTOSYNTHESIS 261 



is reduced to carbon monoxide, the latter is converted to formaldehyde 

 which in turn is oxidized to formic acid ; this unites with carbon dioxide to 

 form oxalic acid by a reversal of the familiar reaction C2O4H2 —> CO2 + 

 HCOOH. ' 



The carboxylic acids found in plants which might come under considera- 

 tion as intermediate products in the conversion of carbonic acid to carbo- 

 hydrates under the Liebig theory are: formic, oxalic, glycollic, glyoxylic, 

 tartaric, lactic, malonic, succinic, malic and citric acids. The chemical evi- 

 dence in support of the principle of Liebig's theory, as has been stated, is 

 rather meagre. In this connection may be mentioned Fenton's * reaction 

 of the formation of glycollic aldehyde from tartaric acid. The latter with 

 hydrogen peroxide and ferrous sulfate yields dioxymaleic acid which on 

 warming in water solution loses carbon dioxide and yields glycollic 

 aldehyde.^ 



CHOH.COOH COH.COOH CH^OH 



I > \\ > I +2CO2 



CHOH.COOH COH.COOH CHO 



Tartaric acid Dioxymaleic acid Glycollic aldehyde 



Glycollic aldehyde is easily converted into hexoses. Neuberg and 

 Schwenk ^ have found that dioxymaleic acid is converted into glycolHc 

 aldehyde and carbon dioxide by the action of yeast. Neuberg and Kerb ^ 

 have also found that hydroxypyruvic acid, CHoOH . CO . COOH, by the 

 action of yeast yields glycollic aldehyde and carbon dioxide. Similar re- 

 actions, involving the splitting out of carbon dioxide from the carboxyl 

 group, have been obtained by the action of light.^ Thus, for example, 

 lactic acid yields acetaldehyde and tartaric acid yields glyoxal, pyruvic 

 acid, acetaldehyde, etc. 



While reactions of this nature may play a role in the ripening of fruits 

 and in the deacidification of plants in general, it is difficult to apply them 

 directly to the photosynthetic process. The formation of compounds such 

 as malic or glycollic acids from carbonic acid is difficult to picture. It is 

 more probable that such acids arise from the incomplete oxidation of 

 carbohydrates and are decarboxylated, leaving a residue which can possibly 

 be converted into carbohydrates by the plant. 



Based upon better chemical experience was the suggestion of Erlen- 

 meyer.® He had found that a-hydroxy-carboxylic acids, RCH(OH) 



*Fenton, Jour. Chem. Soc, 65, 899 (1894) ; 67, 48, 774 (1895) ; 69, 546 (1896) ; 

 71, 375 (1897) ; 73, 71 (1898) ; 81, 426 (1902) ; 87, 817 (1905). 



"Fischer and Landsteiner, Ber. chem. Gcs., 25, 2549 (1892). Aleldola, Jour. 

 Chem. Soc., 89, 756 (1906). 



'Neuberg and Schwenk, Biochcm. Zcit., 71, 104 (1915). 



'Neuberg and Kerb, ibid., 53, 406 (1913). 



''Neuberg and Peterson, ibid.. 67, 63, 71 (1914). Spoehr, ibid., 57, 93 (1913). 

 Nef, Ann. Chem., 357, 291 (1907). Locke, Jour. Am. Chem. Sac, 46, 1246 

 (1924). 



* Erlenmeyer, Ber. chem. Ges., 10, 634 (1877). 



