THE ENERGETICS OF PHO lOSYNTHESIS 



73 



Those workers who find the two-vessel method too comphcated, are now in a 

 position to correctly determine the maximum quantum yield in an alkaline 

 medium. To begin with, the quantum yield in 0.2 molar carbonate buffer 

 at 2 atm % CO2 pressure is measured. Then, the same cell suspension is 

 brought into an acid medium and the increase in the manometrically observed 

 light action is noted when the COo pressure is increased from 2 atm % to 

 5 atm %. This experiment can be carried on with the one-vessel method, 

 as the photosynthetic quotient does not change between 2 atm % and 5 

 atm % CO2. We see from the curve of Figure 29 that at 2 atm % CO2 the 

 quantum yield is 0.290 and at 5 atm % CO2 it is 0.345. Thus, when the 

 CO2 pressure is increased from 2 atm % to 5 atm %, the quantum yield 



0.36 



<P 



0.30 



0.24 



0.18 



012 



0.06 



Fig. 29. The quantum yield as a function of the 

 CO2 pressure (Warburg et al.. Zschr. Xaturf.). 



1.0 20 30 40 



COj pressure (atm %) 



50 



increases by the factor 1.19. If the maximum quantum yield in alkaline 

 medium is 0.304, then we have to multiply this value by 1.19 to obtain the 

 quantum yield in acid medium at 5 atm % CO2 



^ = 0.304 X 1.19 = 0.362 

 l/\p = 2.76 



This value for the quantum requirement is nearly the theoretical value (see 

 § 26). According to equation 27, we find for X = 6440 A the very high 

 efficiency of 



0.04 X 6440 

 2776 



= QV-r, 



93% 



To summarize, it has to be borne in mind that under the right experimental 

 conditions the low, nearly theoretical value for the quantum requirement of 

 photosynthesis can be determined in acid medium with the two-vessel method 

 as well as in alkaline medium with the one-vessel method. A most important 

 factor is the maintenance of the right optimal CO2 pressure : this is 5 atm % 

 in acid medium and 2 atm % in alkaline medium. It must be noted that 

 many investigators preferred the alkaline medium and used a CO? pressure of 

 only 0.2 atm % in their experiments. The values they obtained for the 



