434 



BIrgit Vennesland 



reason for diminished sensitivity to orthophenonthroline under anaerobic 

 conditions, but it is surely one of them. The important fact to focus on is 

 that sufficient concentrations of orthophenonthroline will always inhibit 

 photophosphorylation substantially. In this connection, I regard it as un- 

 fortunate that the substituted ureas ore so often used in preference to ortho- 

 phenonthroline. These reagents have a limited solubility, and it may be 

 impossible for that reason to odd them in amounts sufficient to show the in- 

 hibition of the oxygen-evolving step. 



I have tried to cover the field broadly, to elucidate the experimental 

 basis for Warburg's theories about photosynthesis, and the manner In which 

 these theories are applied to interpret the phenomena we study in isolated 

 chloroplosts. We think of the chemical consequences of photon absorption 

 in terms of chlorophyll and substrate first, rather than In terms of chloro- 

 phyll and the respiratory catalysts. The latter substances must function in 

 the necessary back reaction or reoxidotlon. The cycling process in illu- 

 minated grona is visualized in terms of a release of O^ from the activated 

 CO^-substrate, and a reoxldotion of the reduced substrate by the O2. This 

 picture is consistent with the observed behavior in measured light oflhe gas 

 exchange in intact photosyntheslzing Chlorella, and is not refuted, so far 

 as I con see, by any of the facts that have been accumulated with C , or 

 with highly sensitive spectrophotometric measurements, or with any other 

 analytical technique. 



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REFERENCES 



Warburg, O., Burk, D., Schocken, V., Korzenovsky, M., and 

 Hendricks, S.B., Arch. Biochem., 23, 330 (1949). 



Warburg, O., New Methods of Cell Physiology, Interscience, New 

 York (1962), especially p. 625. 



Warburg, O., and Krippahl, G., Z. Naturforsch. , 15b, 788 (1960). 



Warburg, O., Krippahl, G., Schroder, W., and Buchholz, W., Z. 

 Naturforsch., 9b, 769 (1954). 



Bassham, J. A., Adv. Enzymology, 25, 39(1963). 



Warburg, O., personal communication. 



