115 



Eugene Rabinowitch 



nonphotochemical reaction, the cytochrome f^, with a potential 

 of +0.37 V, thus liberating about 8 kcal. of stored energy. 

 This energy does not need to be lost; it seems plausible 

 that, similarly to the electron transfer between cytochromes 

 in the respiration chain, a part of it (about 7 kcal. per 

 electron pair) can be salvaged in a molecule of high-energy 

 phosphate, ATP. 



The second photochemical reaction transfers the electron 

 from cytochrome f_ to the acceptor, X, again storing about 

 0.75 eV of potential energy. 



3. 



Schemes similar to figure 2 — presented vertically, hor- 

 izontally, in zig-zags, on circles or curlicues — have been 

 proposed by several authors in recent years. They have 

 arrived at these schemes in different ways; the following 

 appears to me as the logical sequence of findings leading 

 to it : 



a. Realization that in photosynthesis, four Il-atoms 



(or electrons) have to be transferred "uphill", over 

 a gap of 1.2 V. 



b. Finding that quanta are used in this transfer 

 (Emerson) . 



c. Finding of two cytochromes in chloroplasts, with 

 potentials suggesting their function as "halfway 

 stations" in the transfer of electrons. ( nil 1 ) . 



d. Observations of the oxidation of cytochrome in chlor- 

 oplasts by light absorbed in one "pigment system," 

 and reduction by light absorbed in another "system" 



( Duysens) . 



e. Observation of an enhancement of the photosynthet ic 

 action of light absorbed in "system 1," by light 

 absorbed in "system 2" (Hmerson). 



The last-named line of evidence appears to me particular- 

 ly convincing. This matter will be presented in more detail 

 in Govindjee's paper later in this symposium. 



It is difficult to show that excitation of system 2 

 is by itself insufficient to bring about complete 



