EFFECT OF PREPARATORY REACTIONS ON LIGHT CURVES 1010 



(28.9) P™-- = nA-aA'E!Ao[C02]/(l + A'ICOa]) 



(28.10) ,//-* = (Sk.KEnCO-,])/{l + K[C02]) 



(28.11) PZl: = nkJRlAo 



(28.12) idP/dk*)o = nAo/8 



with n probably equal to 1. 



So far, we have considered the shape of light curves as determined ex- 

 clusively by preparatory reactions on the "carbon dioxide end" of photo- 

 synthesis (the possible rate-limiting factors being the constant of carbon 

 dioxide diffusion, the bimolecular rate constant of carboxylation and the 

 available concentration of the enzyme Ea). Analogous derivations can be 

 made for limiting influences on the "oxidation end," such as, the rate con- 

 stant of diffusion of reductants, the rate constant of their preliminary trans- 

 formations (e. g., of the binding of hydrogen to an acceptor) and the de- 

 ficiency of enzymes catalyzing these reactions (e. g., the hydrogenase). 

 In making these derivations, we could, for example, set the rate of photo- 

 synthesis proportional to the concentration of the primary oxidation sub- 

 strate such as the hypothetical "bound water," A'H20 or, more generally, 

 A'HR (instead of to the concentration of the primary reduction substrate, 

 ACO2, as we have done so far). However, we abstain from a detailed dis- 

 cussion of these possibilities, because, in the case of green plants, there is no 

 positive proof that a dark hydration reaction actually is needed to make 

 water available for the photochemical process. The abundance of water in 

 cells may make this hydration, even if it were needed, practically instan- 

 taneous. In the photosynthesis of purple bacteria , preliminary transforma- 

 tions of reductants are known to occur, but no definite proof has as yet been 

 given that these transformations must be considered as preparatory reac- 

 tions {i.e., reactions providing the oxidation substrate for photochemical 

 process) rather than as finishing reactions removing the primary oxidation 

 products, formed by the photochemical oxidation of water. (The second 

 alternative is favored by van Niel, Gaffron and Franck; cf. Vol. I, p. 168.) 

 It must, nevertheless, be borne in mind that the rather detailed considera- 

 tion of the preparatory processes "on the reduction side," and the compara- 

 tive neglect of the analogous processes "on the oxidation side" of the pri- 

 mary photochemical process, which is common to most discussions of the 

 kinetics of photosynthesis, are not justified, being based only on our in- 

 ability to study the fate of water before its oxidation in photosynthesis, and 

 our present insufficient knowledge of the initial transformations of hydro- 

 gen and other reductants used by bacteria. 



