PHOTOSYNTHESIS XOT A HOMOGENEOUS REACTION 



865 



violet light {cf. figure 22.35, p. 698)— the rate of light absorption is likely 

 to be much slower for chlorophyll molecules situated deep in the body of a 

 chloroplast (or on the "shady side" of it) than for those located on the light- 

 exposed surface. Thus, inhomogeneity cannot be avoided even by using 

 dilute cell suspensions in which illumination is the same for all cells, but 

 not for all chlorophyll molecules in them. In denser suspensions, only a 

 uniform time average of illumination of all cells can be achieved, and this only 

 by very intense stirring. In the thalli (^f multicellular algae, or in the 

 leaves of the higher plants, the disparity between the rates of light absorp- 

 tion in different cells cannot be corrected at all. The absorption in the 



10 20 30 40 50 60 70 80 90 100 

 TRANSMISSION, % 



Fig. 26.5. Weakening of light in transmission through a leaf of Cyclamen persicum 

 (after Srhanderl and Kaenipfert 1933). Scale at right of leaf cross-section, depth in 



microns. 



spongy parenchyma cells, for example, is under all circumstances consider- 

 ably weaker than that in the paUsade cells (cf. fig. 26.5). Thus, in curves 

 representing the rate of photosynthesis (P) as a function of carbon dioxide 

 concentration or light intensity, the abscissae are mean values (averaged 

 over the whole cell or over many cells). This alone must prevent these 

 cur^^es from following the course suggested by Blackman, even if the law 

 of limiting factors were exactly valid for the ideal case of a uniformly illum- 

 inated and uniformly supplied homogeneous system. Katz, Wassink and 

 Dorrestem (1942) derived an equation by means of which the experimen- 

 tally obtained "light curves," P = /(7o) (where h is the intensity of the 

 light falling on the front wall of a vessel with a suspension of algae or bac- 



