610 LIGHT AND LIVE 



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 with 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 excitation energy by recurrent 

 absorption of photons. The stepwise interaction of the "expelled" 

 electron with the intermediate electron acceptors constitutes the energy 

 conversion process in photosynthetic phosphorylation. Because of 

 the cyclic path travelled by electrons that are activated by light, this 

 type of photosynthetic phosphorylation has been called cyclic photo- 

 phosphorylation (18, 9) . 



Chlorophyll can, of course, also be restored to the ground state 

 when the excited electron returns directly without going through the 

 enzymatic "energy transformer stations," but in that case electron 

 energy has not been converted into chemical energy and hence photo- 

 synthesis has not occurred. Instead, a light quantum is emitted and 

 the characteristic fluorescence of chlorophyll is observed. 



The primary photochemical reaction in which an absorbed light 

 quantum "excites" a chlorophyll molecule and "expels" an electron, 

 is represented by Equations 3 and 3a. The symbol [Chi] + is intended 

 to denote that the chlorophyll molecule as it loses an electron be- 

 comes oxidized or forms a "hole" ["odd ion", (91)] that is ready to 

 accept another electron and to return in this way to its normal ground 

 state. 



Chi + hy -^ Chi* {3) 



Chi* r= [Chl]+ + e- {3a) 



In the proposed mechanism of cyclic photophosphorylation it was 

 suggested (9) that [Chl]+ is restored to its ground state by accepting 

 an electron from a cytochrome present in the photosynthetic particle. 

 This "terminal" cytochrome component, i.e., a cytochrome that is 

 adjacent to, and interacts with, the excited chlorophyll molecule, be- 

 comes oxidized after donating an electron to chlorophyll. We have 

 visualized (9) that phosphorylation is coupled with the oxidation of 

 the terminal cytochrome, in a manner analogous to the phosphoryla- 

 tions which accompany the oxidation of cytochromes by oxygen in 



