THE ACTION OF LIGHT 565 



chlorophyll is oxidised by such reagents as hydrogen peroxide or potassium per- 

 manganate, an aldehyde is formed. Similar results were arrived at by Warner 

 independently. Both investigators also found that an oxidising substance is 

 produced at the same time as the aldehyde. This oxidising agent is of a peroxide 

 nature, and Warner makes the statement that the bleaching of chlorophyll is 

 due to hydrogen peroxide, as had. been stated by Usher and Priestley previously. 

 Wager, on the contrary, was unable to obtain any of the usual reactions of 

 hydrogen peroxide ; but the experimental results of Usher and Priestley are 

 difficult to interpret otherwise. They coated a plate with gelatine containing the 

 enzyme, catalase, obtained from the liver. This enzyme decomposes hydrogen per- 

 oxide with evolution of gaseous oxygen, and so far as is known, does not so 

 decompose other peroxides. There is, however, another enzyme, peroxidase, 

 which decomposes other organic peroxides, as well as hydrogen peroxide, but 

 does not cause the production of gaseous oxygen, so that it could not account for 

 the following result. The film of gelatine, containing catalase, was coated again 

 with a film of chlorophyll, and exposed to light in the presence of carbon dioxide. 

 The gelatine film, after a time, was found to be full of bubbles of gas, while the 

 chlorophyll remained unbleached. The fact can only be explained by the rapid 

 diffusion of hydrogen peroxide into the gelatine film, and its decomposition there 

 before it had time to affect the chlorophyll to any perceptible extent. 



These results are very suggestive in view of what has been said above with 

 regard to the phenomena of optical sensitisation, in which we find frequently a 

 similar activation of oxygen, associated with the partial decomposition of the 

 sensitiser. Although it appears that the product of the action of light on a dye 

 acts, in the ordinary cases, as a catalyst, it does not seem necessary to assume 

 this in the case before us, since the results can be explained by the oxidation of 

 part of the pigment by the peroxide produced, with the production of an aldehyde. 

 At the same time, there is no reason to deny the existence of a catalytic process 

 as part of the phenomenon ; in fact, that part with which we are now concerned 

 is not one in which energy is stored. 



The point to which we have arrived seems to be this. The aldehyde which 

 is split off from the chlorophyll system must have been derived from some 

 constituent of the chlorophyll, probably the phytol, since it is formed by light 

 irrespective of the presence of carbon dioxide. The experiments just related, 

 therefore, give us no information as to the most difficult part of the problem, 

 namely, how formaldehyde is produced from carbon dioxide. 



As to this, Hoppe-Seyler (1881, p. 139) suggested the hypothesis that chloro- 

 phyll combines with H 2 CO 3 ( = CO 2 + H 2 O) ; this compound is supposed " to fall 

 apart, under the influence of light, in such a way as to yield chlorophyll (or the 

 catalyst contained therein), oxygen and a third product, either sugar or a substance 

 from which sugar may be formed." We have seen, on the other hand, that the 

 main process, as a whole, is not a catalytic one, and the assumption of a chemical 

 compound of chlorophyll with carbon dioxide is, at present, devoid of proof. 

 We may perhaps regard the association of carbon dioxide and water with chloro- 

 phyll as some kind of a physico-chemical system analogous to that of muscle. 

 By the taking up of light energy, this system is converted into one in which the 

 chemical potential of the carbon dioxide plus water is raised to that of formalde- 

 hyde plus oxygen, or hydrogen peroxide. The formaldehyde may possibly be 

 combined chemically with the chlorophyll, or it may be merely associated in some 

 way, such that it is split off by the subsequent action of light and oxygen. Of 

 course, the processes proceed simultaneously under natural conditions. We may 

 represent it thus, in diagram, putting (C) for chlorophyll : 



H 2 0@CO 2 + light energy ^@CHOH + O 2 ( 1 ) 



@CHOH Hg) + CHOH (2) 



It must be assumed that the oxygen is molecular, inactive, oxygen and given off 

 as gas, so that the formaldehyde escapes oxidation. The first stage does not appear 

 to have been obtained outside the living leaf and it probably requires the complex 

 mechanism of the chloroplast. What this mechanism is requires further investigation. 



