248 INTERMEDIATES IN REDUCTION OF CO2 CHAP, 10 



plicity of the reaction sequence : 



(10.2a) CO2 > CO + ^ O2 



(10.2b) CO + 2 H > HCHO 



(10.2c) 6 HCHO > CUuOi 



compares favorably with the compUcated system of reactions needed to 

 produce sugars through the intermediary of organic acids. Scheme 10. 1 

 serves to illustrate the theories of Liebig and Baeyer. Arrows pointing 

 downwards correspond to reduction (hydrogenation) ; arrows directed 

 to the right and upwards, to the addition of carbon dioxide (carboxyla- 

 tion) ; while a horizontal arrow means polymerization or condensation. 



The column in which a compound stands in scheme 10. 1 indicates its 

 carbon chain length, nc, while the horizontal level determines its degree 

 of reduction, L. Carboxylation increases nc by 1, and reduces L in the 

 ratio nc/{nc +1). 



Baeyer's theory corresponds, in scheme 10. 1, to the path along the 

 left side down to the middle, and thence horizontally to glucose. Liebig's 

 path goes zigzagging through the field of acids, until it reaches the apex 

 of the table. 



The compounds listed in scheme 10. 1 are only selected examples, 

 since many other isomers can be formed (including unsaturated ones) 

 by keto-enol transformations, hydrations, dehydrations, and dismuta- 

 tions. In chapters 7 and 9, we have repeatedly postulated that dismu- 

 tations play an important part in photosynthesis. A similar suggestion 

 was first made by Baur in 1913. He assumed that light is used in 

 photosynthesis only for the reduction of carbon dioxide to oxalic acid, 

 while the reduction of the latter compound to carbohydrates is brought 

 about by dismutations. 



B. Low Molecular Weight Compounds in Green Plants * 



1. Review of Analytical Data 



If one assumes that some intermediates (or their derivatives) accumu- 

 late, in the course of photosynthesis, in analytically recognizable quan- 

 tities, one could expect help in their identification from ordinary chemical 

 analysis. However, no significant progress has as yet been achieved in 

 this way. This is not to say that compounds with a composition inter- 

 mediate between carbon dioxide and the carbohydrates have never been 

 found in green plants. The trouble is rather, that too many of them 

 are present, and that none can be definitely associated with photo- 

 synthesis. To illustrate the variety of low molecular weight compounds 

 found in green leaves, we have listed in table 10. 1, most organic com- 



* Bibliography, page 273. 



