1118 THE LIGHT FACTOR. II. QUANTUM YIELD CHAP. 29 



Franck (1949), in offering an explanation of Warburg and Biirk's results 

 in terms of photochemical half-way reversal of respiration, suggested that 

 the extent to which this process occurs depends on the capacity of respira- 

 tion intennediates (which probably are organic acids) to penetrate from the 

 protoplasm into the chloroplasts, and that this capacity is affected by the 

 physiological state of the cells. It remains to be seen whether this explana- 

 tion can suffice to explain why many careful experiments have failed to 

 show the existence of the phenomenon. Thus, Emerson and co-workers 

 never had observed any curvature of the light curves in the region of the 

 compensation point, which would indicate a lower quantum requirement in 

 very low light. Brown and co-workers (1950) found no evidence that 

 light interferes with respiration in mass-spectrographic experiments: The 

 uptake of 0(16)0(16) from the air continued in light, while 0(16)0(18) was 

 evolved simultaneously by photosynthesis from algae suspended in 0(18)- 

 enriched water. It was mentioned before (page 1108) that Warburg and 

 co-workers (1949) arrived at a similar conclusion by observations of the rate 

 of oxygen consumption in darkness and light under conditions assuring 

 rapid removal of respiratory carbon dioxide from the medium; it was, how- 

 ever, suggested that these findings might have been contingent on the inter- 

 mittency of illumination, which prevented the utilization for photosynthesis 

 of a large proportion of respiration products. 



To sum up, the possibility of photochemical utilization of respiratory 

 intennediates remains controversial, and the effect of this re-utilization 

 on the quantum requirement in weak light, an open question. Suggestive 

 experimental evidence is available for both a negative and a positive answer. 

 (On the positive side: Warburg's cyanide experiments, Kok's and van der 

 Veen's broken light curves, Calvin's carbon tracer experiments. On the 

 negative side: Emerson and Lewis' smooth light curves, Warburg's experi- 

 ments in C02-free medium. Brown's respiration study with oxygen iso- 

 topes.") Whether Franck's suggestion, that the chloroplasts sometimes 

 are and sometimes are not permeable to respiration intermediates formed 

 in the cytoplasm, can explain these contradictions is uncertain. A knowl- 

 edge of the relative contribution of chloroplasts and cytoplasm to total 

 cell respiration in the dark would be useful in this connection; but no esti- 

 mate of this relation has as yet been made. 



2. Nonmanometric Measurements of Quantum Yield 



The results of nonmanometric measurements of the quantum yield 

 on the whole agree with the lower figures (I/7 = 10 ± 2) found by Emer- 

 son and Lewis, Rieke, and others by manometric studies rather than with 

 the higher figures (7 = 3^^ to 3^4) claimed by Warburg and Burk. 



