408 



LIGHT AND LIFE 



a (at least to a fraction of it) . This could mean that perhaps ulti- 

 mately both steps are sensitized via the two fractions of chlorophyll a, 

 but we can for the present only speculate about the possible operation 

 of such a mechanism. 



It is hard to suppose that the most abundant and the only uni- 

 versally occuring sensitizer — chlorophyll a — could be largely non- 

 functional in photosynthesis. Therefore, rather than conceiving of 

 it as a useless drain of light quanta, we prefer to ascribe functional 

 significance also to the negative effect sensitized by chlorophyll a. 

 An obvious solution is offered by the assumption that both steps are 

 essential, and do cooperate in a cyclic fashion. 



Dr. Ruth Aranow derived some kinetic consequences of such a 

 cyclic mechanism, which is depicted in Fig. 7. Light received by the 

 absorbing state of "P 700" (indicated as P^ed— see below) leads 

 to an excited state (P*) and from there to the bleached form (P^^). 

 Ai and ^2 are rate constants for the first light step as it is activated 

 by the detecting and the activating beam. A-^ and ki bear a similar re- 

 lationship for the production of P^ed by the second light step; kf and 

 kf, are rate constants for eventual dark reactions. These constants 

 were inserted for complete generality and could in special cases have 

 zero value. The derived formula shows the dependency of A/ (the 



red 



'i^ht > P'^- 



kj.kj.k^ 



light 



- ox-*" 



Pred ' P* ' ''ox = ^total 



f f ^ ^I^D ^ 4^a] [^.d] ^ [^ ' S^D ^ ^4'a] f-ox] 



= steady state 



If- P , • l.P.*, then; 

 "• total 1 1 



K * "a^D * '^4'a> Ptot 



'%edj = (kfHkJ^(kj+k,Up4(k2^l<^)l^ 



^I ' ^D '^ ^otal 



Xb " "^3^ " ^i^A 



K ^ S'd 



(kf^kj,) + (k, + kjUp + (Kg - K^ii^ 



rig. 7. Kiiiclics of a cyclic system. 



Tk-ny-nvTkpi^ 



