398 LIGHT AND LIFE 



chlorophyll a: one intimately connected to accessory pigment, ac- 

 tivating photosynthesis (cf. next section), and free to fluoresce; the 

 other connected to a quenching pigment, the concentration of which 

 is so small that its presence is indicated only by a weak fluorescence 

 at longer wavelengths than that of chlorophyll a emission. 



In summary, kinetic and fluorescence data indicate that the key 

 intermediate between the light and the dark steps in photosynthesis 

 might be a j^igment possessing an absorption band in the far red. 



Sensitization of Photosynthesis 



The ubiquitous presence of chlorophyll a in aerobic photosynthesis 

 has designated it as the primary pigment in this process. Occurring 

 with chlorophyll a are always accessory pigments, such as carotenoids, 

 chlorophyll b in green plants, and phycobilins in other plants. The 

 occurrence of two pigments with such similar absorption spectra 

 as chlorophyll a and b in the green plant has long been a mystery. 

 Since the red absorption Ijand of chlorophyll b lies at shorter wave- 

 lengths than the corresponding band of chlorophyll a, energy trans- 

 fer from b to a is possible (and has been shown to occur) . Because of a 

 lack of evidence to the contrary, the accessory pigments have been as- 

 signed the passive role of gathering light energy and transferring it 

 to chlorophyll a. In this picture they have no specific function in 

 energy conversion reactions. 



However, in recent years abundant evidence has come to light to 

 indicate that sensitization of aerobic photosynthesis is more complex. 

 Haxo and Blinks (9) found that in red algae the phycobilins were 

 more efficient sensitizers than chlorophyll a. This phenomenon can 

 be correlated with the earlier observation of Emerson and Lewis (8) 

 that the quantum yield of green plant photosynthesis starts to de- 

 cline beyond 680 m^, in the middle of the chlorophyll a absorption 

 band (and at the long wavelength limit of absorption by chlorophyll 

 b, the accessory pigment of green plants) . 



Emerson and his coworkers (6) later observed that the long wave- 

 length limit of photosynthesis can be extended (i.e., the apparent 

 efficiency of chlorophyll a can be increased) by simultaneotis illumi- 

 nation absorbed l)y an accessory pigment. It was shown that this 

 so-called enhancement effect was sensitized by chlorophyll b in green 

 plants and a small fraction of the chlorophyll a (cf. also Blinks (1) 

 and Myers (18)), by fucoxanthol in brown algae, and by phycobilins 

 in blue and red algae. 



Various workers seem to agree that the rate of photosynthesis 



