103 



James Franck and J. L. Rosenberg 

 NATURE OF THE TWO PHOTOCHEMICAL ACTS 



For the application of a pictiire in which singlet and meta- 

 stable state energy of chlorophyll a are used for the two photo- 

 chemical steps of photosynthesis, we have to be more specific 

 about these steps. From the work of Hillv^) and Kamenv^j it is 

 known that cytochrome f (designated as Cyt-^ed ^^^ ^y^ox ^°^ ^^^ 

 ferro- and ferri-forms, respectively) plays a role in the photo- 

 chemistry. If the light reactions consist of water-splitting, 

 the cytochrome is regarded in our scheme as the primary OH 

 acceptor while the H acceptor designated as A, is the primary 

 photosynthetic oxidant. From the determination of the standard 

 electrode potential of Cyt in the chlorophyll -cytochrome complex 

 (Chl-Cyt) by Kok(5)^ we know that the Cytox cannot evolve oxygen 

 by dark reactions alone. Therefore a second photochemical step 

 is needed on the Oh side by which CytQ^ is reduced and another 

 enzyme Y is oxidized to a state capable of yielding oxygen in 

 subsequent dark reactions. Both Cyt and Y must be complexed with 

 the exposed chlorophyll. The scheme is summarized in the fol- 

 lowing set of essential chemical reactions, where (l) and (2) are 

 reactions of photo-excited Chi molecules, the first in the 

 metastable state and the second in the excited singlet state, 

 (la) and (2a) represent sequences of enzymatic dark reactions. 



A + (Chi* .-Cyt J -> AH + (Chl-Cyt ) (l ) 



^ met red ox 



2 AH -> AHg + A (la) 



Y , + (Chl* . -Cyt ) -> (Chl-Cyt ^) + Y ^ (2) 

 red ^ ^^"- sing "^ ox^ ^ "^ red' ox 



Y ^ Y . + ^ (2a) 



ox red 4 2 



GENERAL CONSEQUENCES OF THE SCHEME 



The phenomena described in this section will be first applied 

 to green plants, with omission of the long wavelength absorption 

 (above 680 m\i) . The consequences of the far-red absorption will 

 be considered later. 



Since the exposed chlorophyll comes in contact with many kinds 

 of diffusible substances, a possible competition may exist for the 



