1042 BIOLOGICAL EFFECTS OF RADIATION 



the carbon dioxide (or its rearrangement product) is not accomplished 

 directly through the splitting off of oxygen, but rather through the 

 formation of water (Fig. 9). 



d. The second photochemical reaction is the hydrogenation of 

 chlorophyll and formation of a hydrogen donor. It is known that 

 chlorophyll can hold a small amount of water with great avidity. It is 

 assumed that this combined water is decomposed through the action of 

 light: 



H2O -^ OH(H202) + H. 



The chlorophylls are easily partially hydrogenated without affecting 

 their absorption spectrum. It is assumed that herein the point of attack 

 is at carbon atoms 9 and 10 in the chlorophyll molecule. 



e. The hydrogen peroxide formed in the reaction shown above is 

 broken down in the cell by catalase, with the liberation of oxygen. 



Indication of labile addition compounds of chlorophyll with carbon 

 dioxide, carbon monoxide, and oxygen has been obtained by Padoa 

 and Vita (88) who observed shifts in the spectral absorption bands of 

 solutions of chlorophyll when treated with these gases. 



The fact that chlorophyll contains an easily dehydrogenated group 

 has been utilized by Conant, Dietz, and Kamerling (13) as the basis of a 

 theory of the mechanism of the photosynthesis. They suggest that the 

 first step is the reduction of carbon dioxide by chlorophyll itself, the latter 

 being converted to dehydrochlorophyll. This step would involve the 

 action of an enzyme and would constitute the Blackman reaction. The 

 regeneration of chlorophyll from dehydrochlorophyll would require 

 energy and this is assumed to occur in a photochemical reaction. Accord- 

 ing to this view, the reduction of carbon dioxide proper would take place 

 in the dark and only the regeneration of chlorophyll from dehydro- 

 chlorophyll would involve a photochemical reaction. As StoU (114) 

 points out, however, with this formulation, the dehydrochlorophyll would 

 be expected to be the more stable form and this has never been found in 

 leaves. Another modification of the Willstatter and Stoll theory has 

 been proposed by Shibata and Yakushi (105, 151) in which water is 

 added to the carbon dioxide-chlorophyll complex on the magnesium. 



An interesting case of a modified form of photosynthesis is presented 

 by the purple and green sulfur bacteria which have recently been investi- 

 gated by van Niel (129, 130). These organisms require hydrogen sulfide 

 and light for their development. The purple forms can also utilize 

 elementary sulfur, sulfites, and thiosulfates, and oxidation products of 

 these compounds, and reduction products of carbon dioxide result. 

 Oxygen is not liberated by either the green or the purple forms. The 

 net result may be expressed : 



