SOME GENERAL KINETIC CONSIDERATIONS 867 



should occur whenever the over-all rate of photosynthesis approaches the 

 maximum rate of carbon dioxide supply by diffusion. In this considera- 

 tion, the diffusion of carbon dioxide could be replaced bj^ a preliminary 

 chemical reaction the rate of which is proportional to the concentration, 

 [C02],e-g-, the formation of the compound !C02} from carbon dioxide and 

 an "acceptor," which was postulated in chapter 8 (Vol. I). In this case, 

 light saturation is determined by the maximum possible rate of formation 

 of {CO2} that is reached when all acceptor molecules are free, (i. e., when 

 all complexes { CO2 1 are utilized for photosynthesis practically instantane- 

 ously after their formation). Similarly, "carbon dioxide saturation" must 

 occur whenever the over-all rate of photosynthesis approaches the rate of 

 supply of light energy, and the quantum yield assumes its highest possible 

 value. 



There are other factors, besides carbon dioxide supply and the supply 

 of light energy, which also can impose "ceilings" on the over-all rate of pho- 

 tosynthesis and thus cause saturation phenomena. This role can be played, 

 e. g., by the concentration of any one of the several catalysts participating 

 in photosynthesis (including the "photocatalyst" chlorophyll). For ex- 

 ample, if one reaction step in the "catenary series" of photosynthesis is the 

 monomolecular transformation of a catalyst-substrate complex: 



(Catalyst + Substrate) > Catalyst + Product 



the maximum rate of this reaction step is reached when all the available 

 catalyst molecules are loaded with substrate molecules. When photosyn- 

 thesis proceeds at a rate that requires such maximum utilization of one cata- 

 lyst, variations in most kinetic variables (such as the concentrations of the 

 reaction partners or of the other catalysts, or light intensity) cannot increase 

 the rate any further. Only a rise of temperature can lift the "ceiling" im- 

 posed by the maximum velocity of such a catalytic transformation. The 

 part of a "rate-limiting" catalyst can be assumed by any of the several 

 catalysts the existence of which was postulated in Volume I {cf. chapters 

 6, 7 and 9) — for example, the "carboxylase" Ea, the "stabihzing catalyst" 

 (a "mutase" ?) Eb, or the "deoxygenases" Ec and Eq. Chlorophyll can 

 play a similar role, c. g., if the primary photochemical process involves a 

 chemical change of this pigment, and a certain time is required for its restor- 

 ation. "Acceptors" or "carriers," such as the carbon dioxide acceptor 

 postulated in chapter 8 (Vol. I) are catalysts, too, and the available quan- 

 tity of any such auxiliary compound also can serve as a "limiting factor" 

 in photosynthesis. 



All these factors can — and most of them probably do — contribute to 

 the limitation of the rate of photosynthesis under different conditions, thus 

 causing the "saturation" of this rate wath respect to vacious kinetic vari- 



