INTERPRETATION OF OARRON DIOXIDE CURVES 033 



(/ = 0, and thusfc* = 0). We will return to the evaluation of K^ from car- 

 bon dioxide curves in section g. 



(/) Acceptor "Blockade" 



In comparing the equations obtained in the preceding sections, with 

 the empirical carbon dioxide curves, one has to keep in mind that, despite 

 the considerable complexity of some of these equations, they all embody 

 certain simplifjdng assumptions, and therefore cannot be valid except 

 within a Hmited range of conditions. 



The only kinetic factors taken into account so far were slow diffusion 

 of carbon dioxide, slow carboxylation and limited quantities of the acceptor 

 A and of the carboxylase Ea. In section e we discussed the additional com- 

 plications that may be caused by the deactivation of the primary photo- 

 chemical product, HX-Chl-Z, in competition with its reaction with ACO2, 

 or by the accumulation of the photosensitive complex in the reduced form, 

 HXChl-HZ. 



A single slow preparatory dark reaction with a rate proportional to 

 [CO2] plus the limited quantity of a single catalytic agent (e. g., Ea, A or 

 Chi) could suffice to account for the increase of P at low values of [CO2], 

 for the individual saturation of each carbon dioxide curve and for the oc- 

 currence of "absolute" saturation (i. e., saturation with respect to both 

 [CO2] and /). We know, however, that several catalytic steps of limited 

 maximum efficiency play a part in photosynthesis ; and, although some of 

 these steps are not directly associated with the assimilation of carbon 

 dioxide, a limitation of the rate of the over-all reaction, whatever its source, 

 must be reflected in the shape of the carbon dioxide curves, particularly in 

 the region where they approach "absolute saturation." 



In Volume I, we outlined a general scheme of photosynthesis that in- 

 cludes, in addition to preparatory supply reactions, two other main types of 

 catalytic processes — "finishing" reactions, associated with the conversion 

 of the first reduction products into carbohydrates and with the production 

 of molecular oxygen. 



What happens to the first reduction product (which we will now desig- 

 nate as AHCO2) can affect the rate of photosynthesis in various ways. 

 If, for example, this product has to undergo a chemical transformation 

 before it can be separated from the carrier A, this transformation may re- 

 ciuire a certain time, so that, in intense light, a considerable fraction of the 

 acceptor A can be "blocked" by AHCO2. Or else, the product AHCO2 

 may require a catalyst (^b) for its stabilization, and unless this catalyst is 

 available within a sufficiently short time, AHCO2 may react back (with 

 the oxidized photosensitive complex X-Chl-Z, or with the intermediates 



