268 PHOTOSYNTHESIS 



of this chapter. The attempts which have been made to determine whether 

 there occurs a sphtting of carbon dioxide into oxygen and carbon monoxide 

 are described in section 4 of this chapter. 



Baeyer's assumption of the close analogy between chlorophyll and 

 hemoglobin and the property of the former to absorb carbon monoxide is 

 erroneous. This in itself is, however, not a serious objection. Whether 

 carbon monoxide is actually formed is still an open question. The fact 

 that carbon monoxide cannot replace carbon dioxide in photosynthesis and 

 that the former is poisonous to plants in larger concentrations cannot be 

 taken as an argument that no carbon monoxide could be formed in photo- 

 synthesis as Krascheninnikofif ^^ has pointed out. 



A far more serious question in Baeyer's original theory is the mode 

 of reduction of carbon monoxide to formaldehyde. Baeyer's statement is : 

 "Es braucht nur Wasserstoff aufzunehmen." Undoubtedly formaldehyde 

 can be formed in this manner, but is there actually hydrogen available 

 and what is the source of the hydrogen? Many authors including Berthe- 

 lot, Boussingault, Liebig, Baeyer, Brunner and Chuard make the tacit 

 assumption of the availability of active or "nascent" hydrogen ; few have 

 gone into the question as to whether any support can be found for such 

 an assumption. 



It has been assumed that water is decomposed into hydrogen and 

 oxygen by the action of light ; the oxygen escapes and hydrogen is taken 

 up by the carbon monoxide. Bredig ^^ also suggested that the hydrogen 

 is absorbed by a substance having properties in this respect similar to 

 palladium which then catalytically reduces carbonic acid. This is based 

 upon the observation of Bredig and Carter that formic acid is formed 

 when alkali salts of carbonic acid are treated with hydrogen under pres- 

 sure in the presence of "contact substances." As an example of the latter 

 Bredig calls attention to the fact that platinum is capable of dehydro- 

 genating many organic compounds in a manner which is similar to the 

 action of certain enzymes. In the plant it is assumed there exist enzymes 

 which are capable of absorbing hydrogen, resulting from the photochemical 

 decomposition of water. 



In most of the theories -* the decomposition of water has been taken 

 as the source of hydrogen, either directly or according to CO -f H.O -^ 

 CO2 + H2. These all assume that one of the complex of endothermal 

 reactions comprising photosynthesis is the formation of hydrogen from 

 water. We have no evidence that there actually occurs a splitting of water 

 into hydrogen and oxygen in the plant under the influence of light. These 

 assumptions are generally based upon the observations that in ultra-violet- 

 light water is thus broken up, but it is exceedingly doubtful whether this 

 has any application to the photosynthetic process. 



^ Krascheniniiikoff, Rev. gen. Hot., 21, 177 (1909). 



"Bredig, Ber. chcm. Ges., 47, 541 (1914) ; Die Umschau. 18, 362 (1914). 



"Lob, Landiv. Jahrb., 35, 541 (1906) ; Zeit. Elektrochem., 12, 282 (1906). 



