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Photosynthesis /20 : 5 



chloroplasts. The structure of chlorophyll a and its absorption spectrum 

 are shown in Figures 6 and 7. The absorption spectrum shifts to the 

 red on extraction from the whole cell. 



Similar chlorophylls are found in blue-green algae. The chlorophylls 

 in bacteria, however, have different absorption maxima and slightly 

 different structures. Some form of chlorophyll is found in all photo- 

 synthetic organisms. Those in euglena are chlorophylls a and b. 



Carotenoid pigments are involved in photobiology, not only in photo- 

 synthesis but also in vision. As described in the preceding chapter, 

 many vertebrate visual pigments involve a carotenoid derivative called 

 retinene. In addition, the eyes of snakes and birds have carotenoid 



380 420 460 500 540 580 620 660 700 

 Wavelength (mu.) 



Figure 7. Relative extinction coefficient of chlorophyl a in 

 methanol. After D. G. Harris and F. P. Zscheille, Botanical 

 Gazette 104: 515 (1943). Copyright 1943 by the University of 

 Chicago. 



oil droplets which appear to act as filters. A variety of carotenoid 

 pigments are also found in chloroplasts. The general structure of 

 carotenoids is shown in Figure 8, while two absorption spectra are 

 illustrated in Figure 9. The carotenoids apparently act to absorb the 

 photons which the chlorophylls miss and somehow pass the energy on 

 to the chlorophyll. 



The action spectrum, that is, the relative yield of hexose, at constant 

 light intensity is shown in Figure 10. The most striking feature of this 



