CAROTENOIDS AS SENSITIZERS IN GREEN PLANTS 1151 



The sharp dechne in quantum yield, which sets in, according to figure 

 30.1, above 680 mn, will be discussed in section 5. 



Figure 30.1 was obtained with a dense suspension, which absorbed 

 practically completel}' at all wave lengths. Emerson and Lewis also made 

 measurements with a thinner suspension, which transmitted about one 

 half of incident light, to be able to compare directly the "action spectrum" 

 of photosynthesis in Chlorella with the absorption spectrum of the same 

 specimen. Figure 30.2 shows the results. The two curves were drawn 

 to coincide at 660 m^u. They show a significant divergence above 690 and 

 below 570 m/x. The lower position of the action curve in the green, blue 

 and violet illustrates the relative inefficiency of the carotenoids; but the 

 comparative narrowness of the gap between the two curves confirms the 

 conclusion, reached above, that the carotenoids are not entirely inefficient. 



The total absorption was measured directly; "active" absorption was calculated 

 from photosynthesis on the assumption that all light used for photosynthesis gives a 

 quantum yield of 0.084. This value was chosen to give agreement between the two 

 curves in the red, where all absorption is assumed to be active. The half widths of the 

 bands used are shown on the figure. The cells used in the run covering the red part of 

 the spectrum were from a separate culture. 



The relatively low sensitizing efficiency of the carotenoids of green 

 plants in photosynthesis, indicated by these experiments, may be either a 

 uniform property of all pigments of this group, or it may be an average, 

 e. g., some carotenoids may be as efficient as the chlorophylls, while others 

 are entirely inactive. On the basis of the absorption analysis in figure 22.44 

 it seems unlikely that the inactive fraction of the yellow pigments does not 

 consist of carotenoids at all, but is formed by pigments such as flavones or 

 anthocyanines. 



It can be argued that the shape of the quantum yield curve below 570 

 mju could also be explained by assuming that a complete inefficiencj'' of 

 carotenoids is partly compensated by an enhanced efficiency of chlorophyll. 

 The possible difference between the photochemical functions of chlorophyll 

 in the three excited states (corresponding to the blue-violet, orange and 

 red band systems, respectively) is an important problem. The available 

 evidence gives little indication of such a difference. In chapters 21 (page 

 634) and 23 (page 748) we concluded, from the excitation of the same red 

 fluorescence band by light of all wave lengths, that chlorophyll molecules, 

 excited to the electronic states A or B, are rapidlj^ transferred, by a radia- 

 tionless process, into the lowest electronic excitation state, Y (which is 

 the upper state of the red fluorescence band). However, on page 752, we 

 concluded from Livingston's data, that, in the case of the blue-violet ab- 

 sorption band, the yield of this transformation is far less than 100% — 



