1138 THE LIGHT FACTOR. II. QUANTUM YIELD CHAP. 29 



absorbed light quanta. In "aged" cell suspensions, in particular, the 

 maximum quantum yield can be much smaller than in healthy young cells 

 (cf. fig. 28.13). The reasons for this inactive state are as yet unknown and 

 may lie in nutritional or enzymatic deficiencies (we recall, for example, 

 van Hille's experiments on the revival of photosynthesis in aged Chlorella 

 cultures by a fresh supply of N2) or in the obstruction of catalytic surfaces 

 by narcotizing metabolites ("chlorellin"; cf. page 880). Under the action 

 of external narcotics (cf. fig. 28.9C) the initial slope of the light curves is 

 clearly depressed, i. e., no full quantum yield can be obtained even in ex- 

 tremely weak light. This must be attributed to the inactivation of a cer- 

 tain proportion of chlorophyll complexes, probably by adsorption of the 

 narcotic ; the light quanta absorbed by these complexes remain unavailable 

 for photosynthesis. 



An interesting question— not yet investigated experimentall}^ — is 

 whether a "substandard" quantum yield can be corrected, at least par- 

 tially, by an increase in temperature. If the low yield is caused by some 

 nonphotochemical process that has become so slow as to depress the rate of 

 photosynthesis even in very weak light, heating should accelerate this 

 process and thus improve the yield; but, if the inefficiency is caused by 

 the fact that a certain proportion of the photosensitive chlorophyll com- 

 plexes are inactive (partially decomposed, or obstructed by adsorption), 

 heating might have no effect on the yield. 



Another interesting question is whether, even in the case when all 

 photosensitive catalytic complexes are fully efiicient, the experimental 

 maximum quantum yield must correspond exactly to the number of quanta 

 actually used in the reduction of carbon dioxide. 



This question is raised by consideration of schemes 28.1A,B and 28.11, 

 all of which envisage a competition between stabilizing "forward" reactions 

 and primary or secondary back reactions. Examples of primary back reac- 

 tions are (28.20a', 28.21a' and 28.41a') ; examples of secondary back reac- 

 tions are (28.20d and 28.21d). 



In scheme 28. lA, the "rate-determining" reaction is (28.20b), and the 

 rate is given by equation (28.23). The quantum yield is: 



(29.9) P/Ia = P/k*I Chlo = 7ikr[AC02]/{k' + A-JACOj]) 

 The maximum quantum yield (reached when [ACO2] = Ao) is: 



(29.10) 70 = kr\nn/ik' + AvAo) 



In scheme 28.11, the rate is described by equation (28.42), and the 

 maximum quantum yield is given by equation (28.43) : 



