322 RADIATION BIOLOGY 



moreover, found evidence for the participation of a zinc-containing cata- 

 lyst. It would seem probable that the light energy enters into a redox 

 system of unknown composition (Wassink, 1947) which may be active in 

 accepting energy from the pigment system. The acceptance of energy 

 leads ultimately to the evolution of oxygen. The redox system might be 

 restored in a dark process, supplying it with water or with hydrogen 

 derived from water in a dark reaction. Holt and French (1949, p. 278), 

 using water enriched with 0'\ showed that the oxygen evolved by chloro- 

 plast preparations comes from water. In terms of the mechanism of 

 photosynthesis, given in Sect. 2, the preparations appear to contain the 

 systems (1) and (2), whereas system (3) is damaged or, at least, its 

 connection with the first-mentioned .systems is broken. 



Vishniac and Ochoa (1951) recently succeeded in reestablishing in grana 

 preparations a connection between the system of photochemical oxygen 

 evolution and an enzymatic system for carbon dioxide fixation. The 

 importance of this fact is emphasized by the circumstance that the 

 coupling was achieved by the reversible oxidation-reduction of tri- or 

 diphosphopyridine nucleotide. The authors summarize their findings by 

 stating that illuminated chloroplasts are able to catalyze the over-all 

 reaction 



m 



H2O -|- tri- or diphosphopyridine nucleotide 



^^Oi -\- reduced form of tri- or diphosphopyridine nucleotide. 



They furthermore suggest that chemosynthetic bacteria [cf. the studies 

 by Vogler and Umbreit (1942) on Thiohacillus thiooxidans] may couple 

 the oxidation of the specific substrate to the carbon dioxide fixation 

 through the mediation of pyridine nucleotides. These observations and 

 considerations are in accord with the views outlined previously in this 

 chapter and advocated earlier by the present writer and his collaborators 

 (Dorrestein et al, 1942; Wassink, 1947; Wassink et al, 1942) and also 

 with observations on the connection between phosphate metabolism and 

 photosynthesis, to be discussed later. 



In view of the reduction of various hydrogen acceptors by illuminated 

 chloroplast suspensions, it is of importance that intact algae may carry 

 out similar reactions when reducible materials other than carbon dioxide 

 are supplied. Fan et al. (1943) made experiments along this line. The 

 following considerations, on which their work was based, are of interest 

 in our discussion in view of the scheme proposed in Sect. 2: (1) oxygen 

 liberated in photosynthesis originates from water; (2) some algae are 

 capable of substituting oxidation of a hydrogen donor during photo- 

 chemical carbon dioxide reduction for the oxygen evolution ;» (3) isolated 



3 They mention Gaffron's (1944) experiments with Scenedesmus in which molecular 

 hydrogen was found to act as a hydrogen donor. Another example of the same type 

 is the oxidation of hydrogen sulfide during photosynthesis of some blue algae (Naka- 

 mura, 1938). 



