866 EXTERNAL AND INTERNAL FACTORS CHAP, 26 



teria), can be recalculated to represent P in relation to the average light 

 intensity, /, actually falling on an individual algal or bacterial cell (see 

 p. 1009 and figs. 28.22A, B and C). 



3. Some General Kinetic Considerations 



Some investigators, who realized the inevitable distortion of light curves 

 and carbon dioxide curves of photosynthesis by the "depth effects" dis- 

 cussed in the preceding section, assumed that in the measure in which these 

 effects can be eliminated (experimentally, by the use of very dilute systems, 

 or mathematically, by applying adequate corrections for inhomogeneity) 

 the kinetic curves would approach the ideal "Blackman type." Undoubt- 

 edly, the elimination of depth effects shortens the transitional region be- 

 tween the ascending part of the curves and the saturation plateau; but 

 figure 28.22C shows that it does not make the breaks sharp. Only a frac- 

 tion of deviations from "Blackman behavior" can be attributed to inhomo- 

 geneity; even with all "depth effects" eliminated, the kinetic analysis of 

 photosynthesis will still have to contend with kinetic curve systems of all 

 three types exemplified by figures 26.2, 26.3 and 26.4. 



The common feature of all these curves is the occurrence of saturation, 

 i. e., of states in which the rate of photosynthesis is independent of the vari- 

 able Fi. The saturation level may depend on the parameter F2 — as in the 

 Blackman type and Bose type curve systems (figs. 23.2 and 23.3) — or it may 

 be independent of F2, in which case curve systems of the "third type" 

 (fig. 23.4) are observed. 



(a) Sources of Saturation in PhotosTjnthesis 



The over-all rate of a process consisting of a series of successive chemical 

 or physical stages — sometimes referred to as a catenary series — cannot ex- 

 ceed the rate of any of its individual steps. "Saturation" of such a process 

 with respect to a given kinetic variable, Fi, is therefore reached whenever 

 the over-all rate becomes equal to the maximum rate of a single step (a 

 step which in itself must be independent of this variable). For example, 

 under given conditions of external carbon dioxide pressure and temperature 

 (perhaps also humidity and other factors affecting the colloidal structure of 

 the cell), carbon dioxide can be supplied to the photosynthesizing cells at 

 not more than a certain maximum rate, which is reached when the station- 

 ary concentration, [CO2], at the site of photosynthesis is zero, and the dif- 

 fusion gradient between the medium and the chloroplast has therefore the 

 maximum possible value. This maximum rate of carbon dioxide supply by 

 diffusion is independent of illumination (except for possible indirect rela- 

 tions of the type mentioned on page 863). Therefore, "light saturation" 



