HYPOTHESES OF LIEBIG AND BAEYER 



247 



their presence by chemical analysis or to make it 'plausible by showing 

 that they can be substituted for the normal substrates. Both methods 

 have been attempted in the study of photosynthesis, but without much 



success. 



The first chemical theory of photosynthesis was proposed by Liebig 

 in 1843. He thought that the formation of acids must precede that of 



- . — COa 



Carbonic ocid 



as 



1.0 



1.5 



C, 



C, 



C4 



— HaCO,— 

 Formic acid 



— HaCaQ, 

 > Oxalic acid 



— HaCaOs 



' Glyoxalic acid 



+ GlycoUc acid 

 (Glyoxal) 



HaCQ 



Formaldehyde 



H4CO- 



Mathanol 



— HaCjO, 

 * Mosoxalic acid 



f-* Tartronic ocid 



— H4C3O4- 



•■ Malonic acid 



— H4C3O3— 



■* Pyruvic acid 

 (Olycsric acid) 



H,CaOa^=^-- 



Acefic acid 

 (Gtycolaldehyda) 



— H^CaO 



Acvtaldohyd* 

 (Glycol) 



— H«C303^=^ 

 Lactic acid 

 Glycerald«hyd« 

 (Methylglyoxal) 



— H.CjOa — 

 Propionic acid 

 (Glycerol) 



— H4C4O, 



■■ Dihydroxymaleic 

 acid 



— H4C4O5 



•■ Oxolacetic acid 

 (Tartaric acid) 



— H,C40j 

 * Malic acid 

 (Fumaric acid) 



— H,C404 ' 



*■ Succinic acid 



- — H,C40^^^=^ 

 Totrotes 

 (Acatylacetic ,. 

 ocid) 



— H,oC40» 



Hydroxybutyric 



acid 



Oxalmalonic acid 



— H.CjO, 



► Carboxysuccintc 

 acid 



— HaC.O, 



Citric ocid 



Glucuronic acid 

 — H.aC.O, 

 Gluconic acid 

 — H,oCj05^='-> — H,aC,0, 



Pentoses Hexotet 



— H.CsO, 

 Ketoglutaric acid 



— HioCjO, 

 (-♦Apionic acid 



Chafn growth 



Scheme lO.I. — Low molecular weight intermediates between carbon dioxide and 

 glucose. Compounds in parentheses have the same values of n^ and L, but differ in 

 composition by an H2O group, a difference which does not essentially affect their content 

 of energy. Broken arrows illustrate the reaction scheme of Baeyer. Solid arrows cor- 

 respond to carboxylations. 



sugars, deriving this view from the example of ripening fruits which at 

 first are acid and later become sweet. The common plant acids — oxalic, 

 mahc, tartaric, citric — stand between carbon dioxide and glucose in re- 

 spect to their reduction level as well as to the length of the carbon chains. 

 Liebig's theory remained undisputed until Baeyer suggested, in 1870, 

 that the first stage in photosynthesis is the reduction of carbon dioxide 

 to formaldehyde, and is followed by the polymerization of the latter to 

 sugar. This theory was based on Butlerov's observations of the poly- 

 merization of formaldehyde to formose. Baeyer argued that the sim- 



