144 J. FRANCK 



to diphosphoglyceric acid* if the energy stored in one of the phos- 

 phate bonds is utihzed for the reaction. According to this theory, 

 diphosphoglyceric acid is not the ordinary oxidant of photosynthesis, 

 but it will contribute to photosynthesis when it is present in a high 

 enough concentration as a respiratory intermediate. We believe that 

 this is the case at illumination intensities below the compensation 

 point and, as Dr. Brugger has reported, a corresponding quenching 

 of the chlorophyll fluorescence has been found there (see page 113). 



ADDENDUM 



The theory outlined above has, in the meantime, been useful for a 

 better understanding of certain other observations with photosyn- 

 thesizing cells. On the other hand, two changes in the discussion of 

 the "minor" process of photosynthesis and reduction of Hill reagents 

 seem to be indicated : 



1. This "one quantum process" for the transfer of an H atom 

 from the chlorophyll in its first excited singlet state to the oxidant 

 becomes, in effect, a two-quantum process above a certain low 

 intensity of irradiation, since under those conditions the energy of 

 a second quantum will be utilized for the transfer of one hydroxyl 

 to the enzyme. Only at quite low intensities will a thermal fluc- 

 tuation sufficient for the hydroxyl transfer occur before an excita- 

 tion of the radical takes place (mostly by energy transfer, as in 

 sensitized fluorescence). These deductions are based on observations 

 of the chlorophyll fluorescence and chemiluminescence in living cells. 



2. While reduction of PGA coupled with the transfer of one hy- 

 droxyl to the OH accepting enzyme undoubtedly needs the energy of 

 two quanta per hydrogen transferred, indications are that PGA (and 

 not only DPGA) can quench the chlorophyll fluorescence by utilizing 

 the excitation energy for the transfer of the H atom on Cio to 

 the photosynthetic oxidant. In that case, the heat of activation for 

 the H transfer (and the corresponding loss of energy) must be con- 

 siderably smaller than for the OH transfer. Furthermore, the 

 bond between the hydrogen atom and Cio must be weaker than 

 was anticipated by us. This hypothesis about the heat of activatior 

 is quite plausible. That the bond between the H and Cio is 

 weaker than normal has been deduced quite early by Conant from 

 chemical evidence. 



* A precursor of PGA in the respiratory cycle. 



