1992 EPILOGUE CHAP. 38 



remains to be established; but an energy exchange of this type is definitely 

 known to occur between the accessory pigments, aVjsorbing light of higher 

 frequency (the carotenoids, the phycobilins, and chlorophyll b) on the one 

 hand, and chlorophyll a (bacteriochlorophyll "890" in purple bacteria) 

 on the other hand. The latter two pigments seem to be the only ones di- 

 rectly participating in the primary photochemical process (in higher plants 

 and bacteria, respectively). The spatial arrangement of the pigment 

 molecules, which makes this energy exchange possible (and in some cases, 

 highly efficient), remains to be elucidated; a great puzzle in this field is 

 presented by the low effectiveness of quanta absorbed directly by chloro- 

 phyll a in some red and blue-green algae (as compared to the quanta 

 first absorbed by the phycobilins and then transferred to chlorophyll a). 



The low yield of fluorescence of chlorophyll in vivo (which, however, 

 may be of the order of 1%, rather than 0.1% as formerly assumed, cf. 

 p. 1867)*, puts a rather low upper limit on the life time of excitation, and 

 thus also on the extent of excitation energy migration in living cells. It 

 is nevertheless possible^ — although by no means certain — that this migra- 

 tion is important in the transition from the photochemical process proper 

 (in which a 10~- molar "photoenzyme," chlorophyll, is involved), to reac- 

 tions mediated by the much less abundant (perhaps, 10~'^ or 10~^ molar), 

 non-photochemical biocatalysts. 



The high yield of resonance transfer from accessory pigments to chloro- 

 phyll poses a challenge to submicroscopic morphology — to elucidate the 

 relative position of the molecules of the various pigments in the living 

 state. Are the chromoproteids (phycobiUns), for example, located inside 

 the protein discs, on which chlorophyll (according to the above-mentioned 

 hypothesis) forms a monomolecular adsorption layer? 



Among the problems of reaction kinetics of photosynthesis, that of the 

 "limiting" reaction (or reactions), determining the maximum rate in con- 

 stant light and the maximum yield per flash in flashing light, stands at 

 present in the center of interest. A ratio of 2500 :m between the concen- 

 trations of chlorophyll and of a "limiting" enzyme (with n = 1, or 4, or 8) 

 accounted satisfactorily for the essential features of the instantaneous 

 flash light experiments (such as those of Emerson and Arnold), the maxi- 

 mum yield of such a flash being about one molecule oxygen per 2500 

 molecules of chlorophyll. The time constant of 2 X 10~'- sec, derived 

 from the dependence of the "instantaneous" flash yield on the length of 

 the dark period between flashes, explained well, in conjunction with the 



* New integrating sphere measurements by Latimer (uTipuhlished) gave, for various 

 algae, fluorescence quantum yields extrapolating to 2% for light intensity zero, and 

 increasing to and above 3% in stronger light. 



