QUANTUM YIELD MEASUREMENTS BY THE MANOMETRIC METHOD 1117 



chemosynthesis postulated for hydrogen bacteria (schemes 9. IV) in which 

 two hydrogen molecules reduce carbon dioxide, utihzing the energy lib- 

 erated by the oxidation of four hydrogen molecules by oxygen. 



According to Franck (1949) (c/. page 1115), it is permissible to interpret 

 Kok's results as indicating quantum requirements of 8 (rather than 6) for 

 tme photosynthesis and 4 (rather than 3) for the "low light process." If 

 one wants to retain Kok's picture, one can, for example, suggest that four 

 quanta procUice four oxidation and reduction agents (4HX + 4Z), while 

 the other four produce — by back reaction of another four (HX + Z) pairs — 

 eight HEP-molecules. (The numerical analogy with the hydrogen bacteria 

 would be lost in this way; but it is more plausible that one ciuantum pro- 

 duces a single HX + Z pair than that it jjroduces two such pairs, as was 

 suggested by Kok). 



Van der Veen (1949), in the course of a study of induction phenomena 

 by the thermal conductivity method (chapter 33), found, for tobacco leaves, 

 a light curve of photosynthesis similar to that recorded by Kok — a straight 

 line up to 450 lux, and another straight line, with about half of the slope 

 of the first one, from 450 to 3200 lux. He combined Kok's concept of the 

 reaction mechanism of photosynthesis with scheme 9. IV, interpreting the 

 "energy dismutation" postulated in this scheme {cf. pages 164 and 239, 

 Vol. I), as production of HEP molecules by recombination of a part of the 

 primary photochemical oxidation and reduction products, and "boosting" 

 by these HEP molecules of the reductive power of the remaining reduction 

 products. The specific numbers used in his scheme (eight recombinations 

 to four oxidation-reductions) taken from Kok, could equally well be re- 

 placed by others — e.g., by four recombinations and four oxidation-reduc- 

 tions, as in scheme 9. Ill; and the same is true of the number of quanta re- 

 quired (six, or eight, or even twelve). 



A somewhat different — and perhaps more plausible — interpretation of 

 a comparatively low quantum requirement of "anti-respiration" in weak 

 light has been suggested (Franck, 1949) : This is the (repeatedly mentioned) 

 possibility that intermediates of respiration can be drawn into the photo- 

 synthetic cycle and reduced back to the carbohydrate level, and that a 

 smaller number of quanta is required for this process than for complete 

 photosynthesis. It is important to note that such a half-way interception 

 of respiration would not cause a deviation of the AO2/ ACO2 ratio from its 

 normal value of (approximately) 1 — since the only gas exchange measured 

 in low light will be that due to residual normal respiration (e.g., respiration 

 outside the chloroplasts). Calvin suggested, on the basis of certain C(14) 

 tracer experiments, that a cross-link between respiration and photosynthe- 

 sis exists on the level of malic and oxalacetic acid; however, these observa- 

 tions are still controversial {cf. chapter 36). 



