1970 KINETICS OF PHOTOSYNTHESIS CHAP. 37D 



origin of the quantum yield controversy lies in different experimental find- 

 ings and not, as Warburg asserts, in a conflict between theoretical dogma- 

 tism and "unbiased" experimental approach. 



The basis of Franck's thermochemical considerations is the belief — 

 which most physical chemists will support, but some biologists may refuse 

 to countenance — that the elementary steps in a photochemical and enzy- 

 matic process in vivo must obey the rules known to apply to similar processes 

 in vitro. Not only the law of conservation of energy must be preserved by 

 the over-all reaction — as everybody concedes — but the individual reaction 

 steps must be kinetically plausible. If this postulate is accepted, the con- 

 clusion follows that many steps involved in photosynthesis cannot occur 

 without some energy dissipation into heat. These losses must be added to 

 the net energy requirement of the over-all process of photosynthesis 

 (as illustrated in fig. 29.2) . 



Losses must first occur immediately following fight absorption, in the 

 conversion of the excitation energy of chlorophyll — or another pigment — 

 into chemical energy (potential energy of re-arranged atoms, either in 

 chlorophyll itself or in a sensitization substrate) . 



Fluorescence experiments indicate that all energy absorbed in excess of 

 that of the transition from the (nonvibrating) , lowest excited electronic 

 state to the (nonvibrating) ground state is lost practically instantaneously. 

 (The fact that the quantum requirement of photosynthesis remains con- 

 stant through a large part of the visible spectrum, is in agreement v/ith this 

 conclusion.) All electronic energy in excess of that of the lowest "red" 

 excited state is thus dissipated prior to fluorescent re-emission or photo- 

 chemical utiHzation. The energy of the ->- transition is slightly less 

 than the quantum corresponding to the peak of the red absorption band 

 (located, in vivo, at 675-680 m^); the corresponding energy is 42 kcal/ 

 einstein. 



The next question is: Is the excitation energy of 41 kcal, which we 

 know to be available for fluorescence, also available for photochemical 

 utilization, or are all chlorophyll molecules that escape fluorescence (the 

 yield of the latter in vivo is < 1%) transferred into the metastable (trip- 

 let) state, presumed to exist below the fluorescent (singlet) state {cf. p. 

 790), before they take part in photosynthesis? The evidence for the ex- 

 istence of this state in chlorophyll is indirect (kinetics of chlorophyll- 

 sensitized autoxidations in vitro) ; but the general theory of the molecular 

 states of conjugated double bond molecviles predicts that a metastable 

 triplet state should exist below the lowest singlet state; this conclusion has 

 been verified on many dyestuff molecules. 



