LIGHT ABSORPTION BY PIGMENTS in VIVO 1849 



leaves themselves, are exposed to light, the protochlorophyll band (at 

 635 him) is first replaced by a band at 670 m/x, and only upon longer ex- 

 posure of the leaves is shifted to 678 mju; simultaneously with the latter 

 transformation, the sensitivity of chlorophyll for photoxidation declines. 

 These changes may well be associated with the formation of grana or 

 lamellae, which can be considered as a special type of chlorophyll aggrega- 

 tion, or of chlorophyll attachment to proteins; and in any case, increased 

 resistance to photoxidation does not necessarily mean general loss of 

 photochemical activity, as postulated by Krasnovsky and co-workers. 



We see in Krasnovsky's experiments no reason to abandon the — ad- 

 mittedly speculative^ — picture, developed in section A of this chapter, 

 according to which chlorophyll is present, in green cells, in monolayers 

 interlarded between proteidic and lipoidic layers. We consider it most 

 likely that all of it contributes uniformly to fluorescence, and to photo- 

 synthesis as well. 



Krasnovsky's observations on the shift of the absorption peak in the 

 process of cliloiophyll formation can perhaps be explained as indicating 

 the transformation of "unorganized" chlorophyll-protein complexes into 

 "organized" structures (such as cohesive monomolecular layers), the 

 additional band displacement being due to pigment-pigment interaction, 

 as discussed above in section 3. The spectroscopic difference between 

 the "active" protochlorophyll of etiolated leaves (subject to photochemical 

 conversion to chlorophyll and having a peak at 635 mn, cf. Krasnovsky, 

 Kosobutskaya and Voynovskaya 1953), and "inactive" protochlorophyll 

 in pumpkin and squash seeds (mth a peak at 645-650 m^) can perhaps 

 be explained in a similar way. 



The situation may be different in those red and blue algae in which 

 a large part of chlorophyll appears inactive, and only the part intimately 

 associated with the phycobilins contributes to photosynthesis and fluo- 

 rescence {cf. chapter 30, section 6 and chapter 32, section 6). It would be 

 interesting to obtain evidence of a difference between the absorption 

 spectra of these two fractions. 



(c) Absorption Spectra of Purple and Green Bacteria 



As mentioned in section (a) above. Barer (1953) suspended cells in 

 protein solutions to minimize scattering. The results were particularly 

 satisfactory with bacteria {cf. fig. 37C.28). Barer's absorption curves of 

 the purple bacteria Rhodopseudomonas spheroides, Rhodo spirillum {rubrum, 

 palustris, and capsulatus), reproduced in fig. 37C.29, show (compared to 

 figures 21.30A, 21.30B, 22.26, 22.27 and 22.36) not only a considerable 

 extension of the spectral range, but also the elimination of much of the 



