THE CHEMISTRY OF PHOTOSYNTHESIS 285 



chlorophyll-content on the rate of photosynthesis, Willstatter and StoU 

 have brought evidence to support the idea that there are two separate re- 

 actions involved in the photosynthetic process. One of these is a purely 

 photochemical reaction, the other an enzymatic one. It is therefore as- 

 sumed that the enzymatic reaction consists in the splitting off of oxygen 

 from the chlorophyll-formaldehyde peroxide complex by means of a cata- 

 lase-like enzyme. 



Willstatter and Stoll endeavored to subject their theory to experimental 

 test. They tried to detect the photoisomerization of the chlorophyll-car- 

 bonic acid complex in light by means of a reagent which would serve as 

 a test for a peroxide. Their system consisted of chlorophyll, carbonic 

 acid, peroxidase and pyrogallol. In the presence of a peroxide the pyro- 

 gallol is converted into purpurogallin by the peroxidase. After making 

 special preparations of peroxidase and illuminating their system in an 

 air stream containing 5 to 20 per cent of carbon dioxide, Willstatter and 

 Stoll obtained large quantities of purpurogallin, but this was not the 

 result of carbonic acid reduction, rather of the oxidation of chlorophyll 

 itself, partially induced by light. While all of the experiments directed 

 to establish the formation and subsequent splitting of a peroxide gave 

 only negative results, the positive value of these experiments is to show 

 the extreme complexity of the natural chlorophyll apparatus. The great 

 protection which the chlorophyll in the chloroplast enjoys against photo- 

 oxidation as well as against the splitting out of magnesium through the 

 carbonic acid, indicates that there are conditions in the living chloroplast 

 about which our analytical methods have given us very little information. 



Interesting and suggestive as the theory of Willstatter and Stoll is, 

 it must be realized that it rests upon certain assumptions. The first of 

 these is that formaldehyde is the final reduction product of carbonic acid. 

 Willstatter and Stoll base this assumption upon the fact that the photo- 

 synthetic quotient, CO^ absorbed : O. emitted is unity. It has already been 

 mentioned that the same quotient would be obtained for any comjxDund 

 of the formula CnH^nOn, as glycollic aldehyde, for instance. Another 

 assumption pertains to the absorption of carbonic acid by chlorophyll. 

 While their experiments indicate that this may occur to a certain extent, 

 it still seems somewhat questionable whether this can account for the 

 total absorption of carbon dioxide by the leaf. The possibility of an 

 absorption of carbonic acid by chlorophyll has been considered by a num- 

 ber of workers.^* The investigations of Brown and Escombe, already 

 discussed, have demonstrated that the leaf possesses a very high absorp- 

 tive capacity for carbon dioxide during photosynthesis. This is of a 

 very much greater magnitude than can be accounted for by the absorp- 

 tion of carbon dioxide by chlorophyll, or by any other means which we 

 know of as existing in the leaf. 



" Hoppe-Seyler, "Physiologische Chemie," l^erlin, 1881, p. 139. Hansen, Arb. 

 hot. Inst. IViircburg, 3, 426, 429 (1885). Luther and Hallstrom, Ber. chem. Ges., 

 38, 2288 (1891). Jorgensen and Kidd, Froc. Roy. Soc, 89 B, 342 (1916). 



