352 DANIEL I. ARNON 



photosynthesis) was answered long ago by physicists while physicists find 

 the problem distressingly complicated and therefore uninterestmg" [93]. 



The mechanism of photosynthetic phosphorylation that we have 

 proposed [94] regards the photosynthetic particle, chloroplast or bacterial 

 chromatophore, as a "closed" catalytic system. We have suggested that 

 during the primary photochemical act, one component of the "closed" 

 system, chlorophyll (bound to protein), becomes excited on absorbing a 

 photon and "expels" one of its electrons that has been raised to a higher 

 energy level. The excited chlorophyll thus becomes the electron donor. 

 On losing an electron, chlorophyll assumes a positive charge, and in this 

 way also becomes the electron acceptor in photosynthetic phosphorylation. 



The "expelled" electron returns in a stepwise manner to the oxidized 

 chlorophyll molecule which thereupon resumes its normal ground state. On 

 its return "downhill" path, the expelled electron releases free energy as it 

 passes through several electron carriers. These intermediate electron 

 carriers are coupled w ith enzyme systems that catalyze the phosphorylation 

 process during which electron energy is converted into pyrophosphate 

 bond energy. After returning to chlorophyll, the cyclic journey of the 

 electrons begins once more as chlorophyll molecules acquire fresh excita- 

 tion energy by recurrent absorption of photons. The stepwise interaction 

 of the "activated" electron with the intermediate electron acceptors 

 constitutes the energy conversion process in photosynthetic phosphoryla- 

 tion. Because of the cyclic path travelled by electrons that are activated by 

 light, this type of photosynthetic phosphorylation has been called cyclic 

 photophosphorylation [95, 94]. 



Chlorophyll can, of course, also be restored to the ground state without 

 the excited electron going through the enzymic "energy transformer 

 stations", but in that case electron energy has not been converted into 

 chemical energy and hence photosynthesis has not occurred. Instead, the 

 energy of electronic excitation is emitted as a light quantum and the 

 characteristic fluorescence of chlorophyll is observed. 



The primary photochemical reaction in which an absorbed light quan- 

 tum "excites" a chlorophyll molecule and "expels" an electron, is 

 represented by equations (2) and (2a). The symbol [Chi] + is intended to 

 denote that the chlorophyll molecule as it loses an electron, acquires a 

 positive charge, i.e. becomes "oxidized" or forms a "hole" ("odd ion", 

 [96]) that is ready to accept another electron and to return in this way to 

 its normal ground state. 



Chl + //v^Chl* (2) 



Chi* = [Chl]+ + e- (2a) 



In the proposed mechanism of cyclic photophosphorylation [Chi] + is 

 restored to its ground state by accepting an electron from a cytochrome 



