330 



Govind jee 



VI. CONCLUDING REMARKS AND SUMMARY 



1. The quantum yield of photosynthesis in Chlorella begins to 

 decrease at 690 m|j, and declines to about 507o at 730 mij, (at 10° C) , 

 No analogous decline was noticed in bacterial photosynthesis. 

 This may mean that a photochemical system of type II is absent 



in bacteria. 



2. A prior knowledge of the shape of the "light curve" (the 

 plot of the rate of the O2 evolution as a function of light in- 

 tensity) is necessary to calculate the enhancement factor. A 

 study of this relationship shows the existence of: (a) S -shaped 

 curves, e. g. in photosynthesis under anaerobic conditions, and in 

 O2 evolution in chloroplasts in the presence of ferricyanide with 

 catalytic amounts of DCPIP; (b) curves with a discontinuity or 

 "knick" (700 mjj, light curve for oxygen evolution in Porphyridium ,) 

 (c) "light curves" with a rapidly decreasing slope with increas- 

 ing light intensity, 



3. The Emerson enhancement effect first increases linearly 

 with an increase in supplementary light intensity, then saturates 

 and finally declines. It decreases with an increase in the 

 intensity of far-red light. 



4. The data presented substantiate the hypothesis of two 

 light reactions involved in both photosynthesis and the Hill 

 reaction. From the two action spectra of the enhancement effect, 

 measured with constant far-red and with constant supplementary 

 beam, respectively, and from fluorescence studies, there emerges 

 a picture of two photochemical systems: System I. This is com- 

 posed of Chi a 680/690 and its "trap," P 700. The P 700 is non- 

 fluorescent at room temperature, and fluorescent at liquid nitro- 

 gen temperature. Its fluorescence peak is at 718 m|j.. Some 



Chi a 670 also belongs to this system. System II. This is com- 

 posed of Chi a 670, and the "accessory pigments." Chi a is 

 fluorescent at room temperature. 



5. In Anacystis , measurements, at -196° C, of fluorescence 

 spectra, excited at 436 m|j. (chlorophyll a) and 605 m|a (phyco- 

 cyanin) , show that the relative heights of fluorescence bands 

 at 687 my, and at 718 m|j, are very different. The ratio of the 

 maxima of the 718 m|j, fluorescence band to that of the 687 mjj, 

 band is greater when Anacystis is excited by 436 my. light, than 

 when it is excited by 605 my. light. This suggests the existence 

 of two kinds of chlorophyll a: one that receives its energy 

 primarily from the phycobilins, and another that receives its 

 energy directly from chlorophyll a. 



