SPATIAL DISTRIBUTION OF PICMENTS 717 



hoff's assertion (fig. 22.21) that live Chlordla cells absorb less light than the 

 pigment extract in the region 550-680 mn; they found the absorption 

 of the cells to be the same as that of extracts in the peaks of the bands, and 

 greater every^^here else. 



The weaker absorption by living colls, compared with the pigment ex- 

 tract, in the blue and violet region was noticed by Emerson and Lewis 

 (1942) also in the spectrum of the blue-green alga Chroococcus (cf. fig. 

 22.48B), which is almost free of scattering effects. 



C. Distribution of Absorbed Energy among Pigments* 



The allotment of absorbed light energy to the several pigments is very 

 important for the interpietation of the quantum yield of photosynthesis 

 and, in particular, for the understanding of the role of the accessory pig- 

 ments — carotenoids and phycobihns. 



1. Effects of Spatial Distribution of Pigments in the Cell 



The first step in the apportionment of absorbed energy is separation of 

 the absorption by the "photosjTithetic" pigments— chlorophylls, carote- 

 noids and phycobilins — from that by pigments such as the flavones and 

 anthocyanines, which probably bear no relation to photosynthesis at all. 

 This question w^as discussed before (cf. page 685) ; figure 22.10 was given 

 as illustration of the extreme case of leaves of the "purpurea" variety, in 

 which a very considerable part of incident hght, particularly in the green, 

 is absorbed by the water-soluble red pigments. 



The presence of pigments of this type complicates matters not only by 

 adding new components to the composite absorption spectmm, but also 

 by raising the problem of "color filters": Generally the apportionment of 

 the absorbed light energy to different pigments, in the region of common 

 absorption, requires the knowledge not only of the true absorption curves 

 of the pigments in the state in which they are present in the living cells, 

 but also of their microscopic and submicroscopic distribution. In the 

 case of flavones and anthocj^anines, it is definitely known that their dis- 

 tribution is different from that of chlorophyll— they are concentrated, not 

 in the chloroplasts, but in the cell walls and vacuoles, and thus form "color 

 filters," before or between the chloroplasts {cf. the calculations of Noddack 

 and Eichhoff 1939). This makes it particularly advisable to use, for quan- 

 titative study of photosjTithesis, plants containing as little nonplastid 

 pigments as possible. 



Even if the flavones and anthocyanines are absent, and the object stud- 

 ied contains no separate carotenoid-bearing bodies, it is by no means certain 



*Bibliography, page 738. 



