1012 THE LIGHT FACTOR. I. INTENSITY CHAP. 28 



the rate in strong light. In the case for which figure 28.20 was drawn, the 

 catalyst content could be assumed to be proportional to the content of 

 chlorophyll (in other words, the "assimilation numbers" could be taken as 

 identical), since the two suspensions differed only in quantity and not in 

 quality of the cells. When, however, changes in optical density are brought 

 about by ^■ariations in the pigment concentration ivifhin the cells, the "as- 

 similation number" does not necessarily remain constant. We will deal 

 with these relationships in chapter 32; of the three cases discussed there 

 (umbrophilic and heliophilic plants, cf. figs. 28.16-28.18; green and aurea 

 varieties, cf. fig. 32.2; and normal and chlorotic plants, cf. fig. 32.4), 

 only the last one is characterized by approximate constancy of the as- 

 similation numbers, and thus leads to light curves such as those in figure 

 28.20. In other words, in this case only does the intracellular content of 

 the rate-limiting catalyst change proportionally to the content of chloro- 

 phyll. 



(b) General Shape of Light Curves 



We now leave the effects of inhomogeneity of light absorption and in- 

 quire into the intrinsic shape of the light curves. All kinetic interpreta- 

 tions agree that the initial, almost linear segment of the light curves corre- 

 sponds to a state in which the primary photochemical process is so slow 

 that the catalysts which participate in the nonphotochemical steps can 

 suppl}'^ the substrates needed for, and transform the intermediates formed 

 by, the primary process, without depletion of the former or accumulation 

 of the latter. Only the light curves of purple bacteria generally show a 

 sigmoid-shaped initial part; the probable reason for this was discussed on 

 page 948. 



In the linear section, the quantum yield of photosynthesis has its high- 

 est value along a given light curve. (In sigmoid hght curves, the highest 

 quantum jdeld is in the point where the tangent to the light curve passes 

 through the origin of the coordinates.) 



This yield may correspond to actual utilization of all absorbed light quanta for 

 photosynthesis, or to a certain, not further reducible proportion of quanta wasted by 

 complete or partial inactivity of a certain number of cells, or of a certain fraction of 

 chlorophyll. A similar irreducible loss of energy can be caused by back reactions, if the 

 proportion of photochemical products that they destroy is independent of the rate of 

 formation of these intermediates (cf. page 1037, and chapter 29, page 1137). 



The light curves bend toward the hoi-izontal when the rate of the pri- 

 mary photochemical reaction ceases to be slow compared with the maxi- 

 mum possible rate of one or several of the nonphotochemical processes as- 

 sociated with photosynthesis. As demonstrated in chapter 26, the limit- 



