1304 THE PIGMENT FACTOR CHAP. 32 



ent in a form which is nonfluorescent and photosynthetically inactive. 

 With excitation at 528 m/x (absorbed mainly by phycocyanin) , both phyco- 

 cyanin and chlorophyll fluorescence are emitted and together account satis- 

 factorily for the total fluorescence of the algae. With excitation at 420 m/x 

 (absorbed mainly by chlorophyll a and the carotenoids, and sHghtly by 

 phycocyanin), no chlorophyll a fluorescence band appears, but only a — 

 relatively weak — fluorescence band of phycocyanin, and a broad fluores- 

 cence band of unknown origin (chlorophyll dl) at 720-750 m^u. 



It ma}^ therefore by hypothesized that the "nonfluorescent" and photo- 

 synthetically inactive chlorophyll a is associated with a minor pigment 

 (chlorophyll (P.) which has an even lower excitation level and serves as a 

 "sink" into which the energy of excited chlorophyll a disappears, producing 

 fluorescence of the acceptor (chlorophyll dl), but no photosynthesis. 



A possible reason for the loss of energy conveyed to "chlorophyll c?" is 

 that this pigment is present in a very small concentration. Because of 

 this, energy trapped in its molecules has no chance of further migration, 

 and may have httle chance to reach a "reaction center" (assuming that 

 resonance migration of energy to a reaction center is a necessary step in 

 photosynthesis!). A smaU number of "traps" can be sufficient to catch 

 most of the quanta on their way from chlorophyll a molecules to the reaction 

 centers. 



The spatial arrangement of the phycocyanin and the chlorophyll mole- 

 cules in the cell may be such that the energy quanta absorbed by phyco- 

 cyanin have a better chance to reach the reaction centers (even if they have 

 to pass through some chlorophyll a molecules on the way) , than the maj or- 

 ity of quanta absorbed by chlorophyll a itself. The chemical properties of 

 the phycobilins and of chlorophyll are so different that they cannot be 

 uniformly mixed in the cell (or a cell organ, such as a chloroplast, or a 

 granum). (This puts considerable theoretical strain on any hypothesis 

 which would assume a 100% effective resonance transfer of quanta from 

 the phycobilins to chlorophyll; and yet, such a hypothesis seems to be indi- 

 cated by the fluorescence experiments.) 



The contribution of carotenoids is small, both to the chlorophyll a 

 fluorescence and to the photosynthesis of blue-green algae (c/. chapter 30, 

 fig. 30.10A). 



Similar results were obtained with the red algae, such as Porphyridium 

 cruentum, by French and Young (c/. above p. 1301). In evaluating their 

 (and his own) measurements, Duysens concluded that 80% or more of 

 the quanta absorbed by phycoerythrin are transferred to phycocyanin (the 

 fluorescence of the latter being proportional to the sum of absorption by 

 both pigments). The transfer from chlorophyll to the phycobilins is 

 neghgible (<10%), even for excitation with X 420 m^. 



