ABSORPTION BY CAROTENOIDS IN GREEN PLANTS 721 



jMiich more precise measurements were made b}'- Strain and Manning 

 (1942) with the chlorophyll a -\- c mixture extracted from diatoms, and bj- 

 Manning and Strain (19-43) with the chlorophyll a -i- d mixture from red 

 algae. Figiu-es 22.40 and 22.41 show that chlorophyll c (chlorofucin) 

 contributes about 90% of total chlorophyll absorption at 570 mix; while 

 chlorophyll d account.s for 60% of the total chlorophyll absorption at 470 

 and 90% at 710 m/x. 



These figures arc for methanol extracts. The relative roles of the 

 chlorophyll components in the absorption of light in vivo are uncertain. 

 In the case of chloroph}-!! h, not even the position of its red absorption peak 

 in vivo is kno^vn with any certainty (c/. page 701). It was suggested (cf. 

 page 612) that alternation of the absorption maxima of chlorophylls a and 

 b in tlie red, orange and 3'ellow, best shown by figures 21.1 A and B, may be 

 nature's means to ensure most effective light absorption throughout this 

 region; but all these bands are so broadened in leaf absorption spectra 

 that it is doubtful whether the absorption by the natural mixture a -\- b 

 is, at any wave length above 550 mn, markedly different from what would 

 prevail if either of the two components were alone present in equivalent 

 concentrations. The situation is different below 550 m^i. The region of 

 prevalent absorption by the 5-component, which we noted in extract at 

 450-530 mix (fig. 22.39), in all probability exists also in hve green cells. 

 Because of the "red shift" it probabh^ extends in vivo from about 460 to 

 about 540 m/x. In this region, the presence of chloroph^dl b may be of 

 considerable importance from the point of view of enhanced light absorp- 

 tion by green leaves and algae. 



4. Absorption by Carotenoids in Green Plants 



The distribution of light between the chlorophylls and the yellow caro- 

 tenoids of green plants in the region below 550 m/x has been much discussed. 

 The first estimate was made by Warburg and Negelein (1923). In their 

 calculations of the quantum yield of photosj^nthesis (cf. chapter 29), they 

 decided, from extract spectra, that the carotenoids of ChlorcUa accoimt for 

 30% of the total absorbed light at 436 m/x. 



Figure 22.42 shows the results of the first more detailed estimate by 

 Seybold (1936), based on spectroscopic measurements with an extract 

 from leaves of Phaseolus vulgaris. Figure 22.39 represented similar data 

 for the multicellular green alga Ulva lacluca; this graph shows the absorp- 

 tion by carotene and the carotenols separately (as well as that by the chloro- 

 phylls a -\- b, a and b). The two figures show the absorption by the caro- 

 tenoids becoming noticeable below 530 m/x (in solvents of low polarizabil- 

 ity) and below about 570 m/x in carbon disulfide. The largest percentage 



