1050 BIOLOGICAL EFFECTS OF RADIATION 



intermediate products could then by intermolecular oxidations and 

 reductions build up the higher reduction products found in plants. In 

 this connection an interesting correlation exists between the number of 

 electrons that must be transferred to the carbon and the number of 

 quanta necessary for the process as found by Warburg. 



A further mechanism which would satisfy the energetic relations is 

 that there may be plant products which supply the hydrogen necessary 

 for the reduction of the carbon dioxide at a much lower energy level than 

 water. These compounds may then return to their original state by the 

 reduction of the water which also may be a low energy level reaction. 

 By means of an integration of such low level processes the final result 

 could then be obtained. The participation in these processes of inter- 

 mediate products of the metabolism and respiration of the plant (Spoehr 

 and McGee, 111) may account for the parallelism between respiration 

 and photosynthesis (van der Paauw, 126). 



PHOTOSYNTHESIS IN BACTERIA 



Purple Sulfur Bacteria. — In the recent researches of van Niel, already 

 referred to (129), on the metabolism of the purple sulfur bacteria, 

 evidence has been advanced for a photosynthetic reaction in these 

 organisms analogous to the photosynthetic reaction in green plants. In 

 place of water, these bacteria apparently use hydrogen sulfide as a 

 hydrogen donor for the reduction of the carbon dioxide. Instead of the 

 evolution of oxygen they produce sulfur and in some cases sulfuric acid, 

 though the cultures are all anaerobic. 



It is extremely difficult to reason on the energetics of chemical reac- 

 tions proceeding in such a complicated living system entirely on the 

 basis of thermodynamic data. But in defense of his interpretation of the 

 reduction of carbon dioxide in these organisms van Niel points out that 

 "in order to obtain sufficient oxygen for the oxidation of 12 molecules or 

 H2S . . , the organisms would have to reduce 24 molecules of CO2 . . . 

 by a photochemical process. The quantity of oxygen thus obtained 

 would be sufficient for a chemosynthetic process during which only 1 mol. 

 of CO2 could be reduced chemosynthetically, producing 0.3 mg. of 

 organic matter." By experiment the amount of organic carbon synthe- 

 sized was more than 15 times greater than can be accounted for on the 

 basis of a chemosynthetic process. In this connection it is interesting 

 that the purple sulfur bacteria have an absorption band in the infra-red 

 and that they grow in this region of the spectrum. The quantum of 

 energy here is less than in visible light and this may be associated with 

 the lesser energy necessary for carbon dioxide reduction in this photo- 

 synthetic system. 



The energetics of the photosynthesis of the nonsulfur bacteria studied 

 by Gaffron (31) is also exceedingly complex. These organisms absorb 



