THE LIGHT FACTOR IN PHOTOSYNTHESIS 



1041 



rates at small temperature intervals. His measurements also give some 

 indication that chlorophyll itself is involved in the Blackman reaction. 

 The photochemical reaction, on the other hand, is affected little by 

 changes in temperature or by hydrocyanic acid. 



The exact nature of these reactions has not yet been determined. 

 Willstiitter and Stoll (143) suggested that chlorophyll adds carbon 

 dioxide, that this addition compound through a photochemical reaction 

 undergoes a rearrangement or isomerization resiilting in a peroxide-like 

 compound. The latter breaks down with the liberation of oxygen, 

 catalyzed by an enzyme, and constitutes the Blackman reaction. War- 

 burg and Uyesugi (138) have found some confirmation of this conception 



COO Phy+yl ^.^^^i 



,0H 



COO Phy+yl 



COOCH 



HC — CH 



HjC 



\ 

 CH3 



^2 CHj (IjL- ^^2 K,ni 



Fig. 9. — Structural formulas of chlorophyll a (left) and chlorophyll 6 (right). 



Wiedemann, 119.) 



{Stoll and 



in an investigation of the rates of decomposition of hydrogen peroxide 

 by Chlorella, although the agreement of this with the Blackman reaction 

 is not complete and could not be in view of the fact that Willstatter and 

 Stoll's peroxide is not hydrogen peroxide. 



Recently Stoll (114, 115, 116, 117, 118, 119, 119a,) on the basis of his 

 extensive investigations of the structure of chlorophyll has put forward 

 an elaboration of the hypothesis of Willstatter and Stoll just referred to. 

 According to this view the photosynthetic process is composed of the 

 following steps: 



a. The union of carbon dioxide or a carbon dioxide compound with 

 chlorophyll, resulting in the entry of the carbon dioxide into the molecule 

 of the light acceptor. 



6. The first photochemical reaction, resulting in the rearrangement 

 of the chlorophyll-carbon dioxide combination with the possible forma- 

 tion of a peroxide. 



c. The reduction of the carbon dioxide, or its rearrangement product. 

 This is accomplished through hydrogenation, in which the chlorophyll- 

 carbon dioxide complex is the hydrogen acceptor and a hydrogenated 

 form of chlorophyll is the hydrogen donor. This hydrogenated form of 

 chlorophyll is supposed to arise from a hydrogenation of the chlorophyll 

 at carbon atom 9 in the chlorophyll molecule. Thus, the reduction of 



