INTERPRETATION OP CARBON DIOXIDE CURVES 931 



(such as the possible preUminary enzymatic binding of water) as well as 

 the influence of "finishing" reactions (such as conversion of AHCO2 to a 

 carbohydrate, and hberation of oxygen) have so far been neglected; while 

 the role of reactions within the photosensitive complex proper was taken 

 into account only by assuming the rate of reduction of ACO2 to be equal 

 to the product fc*JAC02], where k% was considered a function of the illu- 

 mination intensity, I. No new source of carbon dioxide saturation was 

 added by this assumption; saturation remained determined entirely by 

 the four factors treated in detail in sections a-d (limited quantity of A, 

 slow diffusion, slow carboxylation and limited quantity of the carboxylase, 



Obviously, however, carbon dioxide saturation can also be produced by 

 limitations of any of the other partial reactions in photosynthesis. For 

 example, msufficient amount of a catalyst needed for the preliminary trans- 

 formation of the reductant (H2O, H2, H2S. . .) might have the same "ceil- 

 ing" effect on the carbon dioxide curves as the limited amount of the en- 

 z>Tne (carboxylase) that catalyzes the prelmiinary transformation of car- 

 bon dioxide itseK. 



Little could be gained by trying to write out equations for carbon di- 

 oxide curves that would include the effect of a Umited supply of the reduc- 

 tant. On the other hand, a few words may usefully be said about the in- 

 fluence on the carbon dioxide curves of a limited supply of light. 



The kinetic equations that follow from the consideration of the prob- 

 able forward and back reactions within the photosensitive complex proper 

 will be derived in chapter 28 (p. 1020 ff.). They show, as expected, that 

 the rate of absorption of light by this complex imposes a limit on the rate of 

 photosynthesis that cannot be raised by increased supply of carbon dioxide 

 (or change in any other external factor). Consequently, the "light factor" 

 is in itself capable of producing a saturation effect in the carbon dioxide 

 curves. For example, if we consider equation (28.14), derived from reac- 

 tion mechanism (28.11), as an equation of carbon dioxide curves (i. e., 

 if we treat 7 as a parameter), we find that, at high [CO2] values, the rate 

 approaches the maximum: 



(27.67) Pmax. = nk*ICh\o 



which is the rate of supply of light quanta multiplied by the number n of 

 carbon dioxide molecules that can be transfomied by a single quantum — 

 perhaps 3^. Half saturation is reached, according to scheme 28. lA, at: 



(27.68) ,/JAC02] = k'/kr 



One could thus ask whether the interpretation of carbon dioxide curves 

 really requires the assumption of an acceptor A, supposed to be present in 



