1962 



KINETICS OF PHOTOSYNTHESIS 



CHAP. 37D 



differed strongly from those of Warburg, Burk et al. A given culture 

 showed the same requirement in repeated experiments, but the require- 

 ments of different cultures varied from 6.1 to 13.5 (quanta per oxygen 

 molecule). Of the various factors studied, concentration of chlorophyll 

 (produced by accidental fluctuations, not by systematic changes in the 

 conditions of culturing), was found to be the most relevant one. Fig. 

 37D.35 shows the decline in quantum requirement with increasing [Chi] 

 value. The median value of all the measurements in this chart is I/7 = 

 8.5; but since the variations appear systematic, and exceed the limits of 

 experimental error, the lowest value found, 6.1, was considered significant. 



10 20 



30 40 50 60 70 

 LIGHT INTENSITY -% 



80 90 100 



Fig. 37D.34. Light curves of photosynthesis derived from the same experiments as 

 in fig. 37D.33, but using slopes determined separately for each point, as in fig. 37D.32 

 (after Brackett et al. 1953i). The curves extrapolate Unearly to zero (no Kok effect). 



Some errors to be watched for in this type of experiments, lie in the relation between 

 straight-light transmission and absorption. Increasing the total pigment content of the 

 vessel by increasing the number of cells in the suspension, can have a different effect on 

 scattering than increasing the amount of chlorophyll in each cell; the calibration curve 

 (fig. 37D.29) should therefore be determined separately for each suspension used. 



{d) Franck's Interpretation of Warburg's 1950-1952 Experiments 



Franck (1953) tried to provide a consistent interpretation of all quan- 

 tum yield requirements, including the 1950-1952 results of Warburg and co- 

 workers. His basic postulate remained the same as in an earlier paper 

 {cf. p. 1117) — namely, that quantum requirements of 4 or less are indic- 

 ative of the substitution of intermediate products of respiration for carbon 

 dioxide as substrate of photochemical reduction. This substitution causes 

 no change in the ratio ACO2/AO2 of the net gas exchange, since the "light 



