PROCESSES IN THE PHOTOSENSITIVE COMPLEX 1021 



have been permissible to take into account only the average ilhimination 

 and to consider the latter as identical for all cells. In other words, the rate 

 of absorption of light by each cell could be taken as equal to the rate of 

 total absorption in the suspension divided by the number of the cells in it. 

 No amount of stirring, however, can mix the contents of the chloroplasts, 

 so that chlorophyll molecules situated deeper inside them always will re- 

 ceive less light than those situated on the illuminated surface. What is 

 even more important, the rate of stirring usually is quite insufficient to 

 make legitimate even the averaging of intensity for whole cells. Such 

 fast stirring is difficult to achieve; it is unlikely that Warburg and Burk 

 liad the right to claim that in their experiments (c/. p. 1006) stirring was 

 so effective that only the average intensity of illumination was important. 

 Often, a danger exists that the periods spent b\' individual cells deep in the 

 suspension, l^etween two exposures to full light in the surface layer, could 

 1)6 long enough to cause induction losses during the subsequent expo- 

 sure (c/. chapters 29, 33 and 34). 



These quahtative considerations show that the way to obtain light 

 curves of photosynthesis best suitable for kinetic interpretation is by using 

 optically thin suspensions or tissues. A limit to this procedure is, how- 

 ever, set by the fact that even single chloroplasts may absorb up to 50% 

 of incident light in the absorption peaks of chlorophyll (cf. fig. 22.35); 

 so that diluting algal suspensions until they al)8orb much less than that 

 amount (or using faintly green tissues such as green onion skins) may 

 merely mean allowing a part of incident light to pass between the chloro- 

 plasts— without improving the uniformity of absorption within the plas- 

 tids. This uniformity can only be improved by employing cells poor in 

 pigment (chlorotic cells), or by employing light which is comparatively 

 weekly absorbed by chlorophyll (e. g., green light). 



We are thus forewarned that the several equations of the light curves, 

 which will be derived below from alternative kinetic models of photosyn- 

 thesis, can be used for comparison with the experimental curves found in 

 the literature, only with strong reservations. We nevertheless consider it 

 worth while to continue with these derivations, as a step toward a more 

 quantitative study of the problem in the future. The latter will require 

 both improved theoretical treatment (including the effects of inhomogene- 

 ous structure and nonuniform light absorption), and, above all, precise 

 kinetic experiments on optically thin objects. 



In analyzing the validity of equations (28.13) and (28.14), two alterna- 

 tive pictures must be considered. According to one, favored by Franck 

 and Herzfeld, the compound ACO2 is part of the "photosensitive complex" 

 proper and its reduction can therefore be considered the primary photo- 

 chemical process, e. g.; 



