122 PROBLEMS OF PHOTOSYNTHESIS 



(39) used bacteria which show luminescence in the presence of Oo traces, a 

 technique that allows the observation of O2 production with the naked eye. 

 At that time, there was no evidence of simultaneous CO2 fixation. It was 

 assumed — without any proof — that Oo was produced by the photolysis of 

 CO2. Later, views were modified. It was admitted that Oo was produced 

 by the photolysis of water and not of COo. The comparative biochemical 

 considerations of Kluyver (34; and van Niel (57), as well as Hill's investi- 

 gations (24, 25) on Oo production of isolated chloroplasts in the absence of 

 COo, provided considerable support for the new hypothesis. In 1946 Nicol 

 (42) wrote that COo photolysis had been accepted as a well-established fact 

 though it was not proven. Stressing this as a typical example of unscientific 

 thinking, he proposed to accept the concept of water photolysis, though at 

 that time the latter was, as it still is to-day, not proven either. We know now 

 that COo photolysis is a fact (see § 49 and § 67). 



In the presence of HoS, certain green sulfur bacteria reduce, upon illumi- 

 nation, COo to carbohydrate to produce .sulfur. 



light 

 CO2 + 2H2S > (CH2O) + H2O + 2 S 



Other pigment-containing bacteria, the purple bacteria, produce sulfuric acid 



light 

 2 COo + HoS + 2 H2O ^ 2 (CHoO) + H2SO4 



If the over-all reaction of photosynthesis of green plants is written as follows 



light 

 CO2 + 2 H2O ^ (CH2O) + H2O + 0> 



a striking resemblance between these reactions is evident. Van Niel therefore 

 proposed to represent photosynthesis by the following general equation 



light 

 CO2 + 2 HoA > (CH2O) + HoQ + 2 A 



The compound H2A is a hydrogen donor; it is H2O in the green plant and 

 H2S in the sulfur bacteria. Organic compounds may also act as hydrogen 

 donors in bacterial photosynthesis, as has been shown by Foster (22) for 

 wo-propanol 



light 

 COo + 2 CHrCHGHCHa ^ (CHoQ) + HoQ + 2 CH3COCH, 



Comparative biochemistry was found by Kluyver and van Niel to be very 

 useful in microbiology. Its application to photosynthesis does not prove that 

 the Oo produced originates from HoO, as the sulfur originates from H2S, 

 but does provide support for this view. 



Experiments with labeled O2 and COo are often quoted as evidence for 

 these purely theoretical considerations.* When a plant has at its disposal 



* See Ruben et at. (48, 49, 50), Kamen and Barker (30), Dole and Jenks (20), Yoshida et at. (77), 

 Winogradow and Teis (75). 



