QUANTUM YIELD MEASUREMENTS BY THE MANOMETRIC METHOD 1109 



It will be noted that this explanation could not be used if Warburg and 

 Burk's concept of all cells being uniformly engaged in photosynthesis 

 throughout the "light period" were correct. More specifically, this ex- 

 planation requires that not only the photocatalytic mechanism responsible 

 for the liberation of oxygen, but also the enzymatic mechanism responsible 

 for the uptake of carbon dioxide, should cease operating within <0.1 sec. 

 after the cells are darkened. This seems to contradict the assumptions 

 which we used on p. 207 in the explanation of the "pick up" of carbon 

 dioxide after intense illumination in C02-deficient medium, on p. 308 in the 

 explanation of the effect of cyanide on yield of photosynthesis in flashing 

 light, and will use in chapter 36 in accounting for C*02 uptake by preillu- 

 minated cells. In all these cases, we have assumed that the capacity to 

 take up carbon dioxide survives, in preilluminated cells, for several seconds 

 (or even minutes) after the cells had been darkened. However, as in many 

 such cases, apparent contradictions may arise from the use of a qualitative, 

 "yes or no" approach, where a quantitative, "more or less" analysis is re- 

 quired. 



Summary of Warburg and Burk's Quantum Yield Measurements. 

 Table 29. IV gives a summary of the quantum efficiencies reported by War- 

 burg and Burk (1950); several of the experiments in this table have al- 

 ready been discussed above. 



Whittingham, Nishimura and Emerson (1951) were able to reproduce 

 Warburg and Burk's results by strict adherence to the same experimental 

 arrangement and schedule of operations. However, they concluded that 

 these results were affected by a sj'-stematic error. Following are the major 

 points of their criticism. 



1. The two-vessel method is very sensitive to slight errors in mano- 

 metric determinations. Thus, a difference of 0.3 mm. in the pressure 

 change registered in one of the two vessels over a 10-min. period may 

 change the calculated oxygen yield by a factor of two. Such a difference is 

 well within the limits of experimental error of the method of Warburg and 

 Burk (as contrasted to the much more precise measurements with the 

 differential manometer, emploj^ed by Emerson and Lewis.) 



2. This low precision of the method leads to random scattering of re- 

 sults, (for example, in experiment No. 7, the I/7 values derived from in- 

 di\'idual cycles scattered from 2.3 to 14). This can be corrected by averag- 

 ing over a sufficiently large number of cycles. However, only in a few 

 experiments of Warburg and Burk, as many as five or six 10 min cycles were 

 used; in most others, only two or three. This explains why even the 

 averaged I/7 values scattered from 2.3 to 4.9. 



3. H,andom errors can explain the scattering of the results, but only a 

 systematic error can explain the consistent finding of I/7 values considerably 



