Introduction 35 



This is very close to the thermodynamically possible minimum value when the 

 molar energy required to split carbon dioxide is 112,000 cal. It is also close to the 

 minimum value that we find with the two-vessel method ; this may appear obvious 

 but is mentioned here because of inexpert criticism of the two-vessel method. 



Since the same number of light quanta absorbed by Chlorophyll develop the 

 same amount of oxygen over the entire visible spectral region, we can calculate 

 the energy transformation in visible light as follows, when we assume that the 

 molar energy required to split carbon dioxide is 112,000 cal.: 



Wavelength of light Energy transformation 



(m/0 



Gain in chemical energy 



Absorbed light energy 



112,000 

 436 2.75^65^00 ^ 10 ° = 62% 



112,000 

 546 2.75 - 52,000 10 ° = ?8 "" 



112,000 



680 TTlTr ^^ 100 = 96",, 



2.75 42,000 



The fact that the quantum yield at a carbon dioxide pressure of 20 mm Brodie 

 represents only a small part of the maximum quantum yield is of historical interest. 

 20 mm Brodie has been the pressure of carbon dioxide at which Emerson deter- 

 mined the quantum requirement of photosynthesis 20 years ago and at which he 

 found a quantum requirement of 10 to 20 per molecule of oxygen. Emerson added 

 the totally unfounded claim that the yield obtained at 20 mm Brodie was the highest 

 possible yield, in spite of the fact that according to our prescriptions Chlorella had 

 been cultivated at a carbon dioxide pressure of 500 mm Brodie. 



Probably no one would have taken Emerson's claim seriously, if Franck and 

 Gaffron 18 had not produced a "thermodynamic theory" to show that the yield 

 Emerson obtained at 20 mm Brodie was indeed the highest that could be achieved. 



In 1949 in Bethesda (U.S.A.), with Dean Burk and several other American col- 

 leagues, we used the newly developed respiration-compensation technique to deter- 

 mine the quantum yield at a carbon dioxide pressure of 500 mm Brodie. In a long 

 series of experiments a quantum requirement of less than 4 was found ; this result 

 agreed with the earlier work done at Dahlem. 



Franck and Gaffron, supported by Eugene Rabinowitsch, stuck nevertheless 

 to their thermodynamics. And without themselves contributing any experimental 

 work to the energetics of photosynthesis, they have been making propaganda for 

 Emerson's Claims for 20 years. 



4. 1-Quantum Reaction 



If we summarize the results of our yield determinations by means of the equation 



H 2 C0 3 + 2.75 hv = CH 2 + 2 



it is evident that this equation does not represent a single chemical reaction but 

 expresses the balance of several reactions. For multiquanta reactions are unknown 

 in photochemistry and no photochemical process, biological or nonbiological, can 



