GENERAL SHAPE OF LIGHT CURVES 1013 



ing influence of a bottleneck reaction in a "catenary series" generally be- 

 comes felt long before the rate of the over-all process has reached the maxi- 

 mum speed of which this hmiting reaction is capable. Consequently, the 

 light curves must approach saturation asymptotically rather than sud- 

 denly "hit the ceihng" (even if we forget for the time being about the ef- 

 fects of inhomogeneity of light absorption, which further enhance the grad- 

 ual character of saturation). For the same reason, the maximum rate 

 reached in the light-saturated state will often be considerably lower than 

 the "ceihng" imposed by the limiting process. 



As to the nature of the processes that can cause hght saturation, the 

 general alternative is between "preparatory" and "finishing" reactions. 

 These two types of dark processes have been fu^st discussed by Warburg, 

 and Willstatter and Stoll, respectively. Because all transformations that 

 occur in photosynthesis must be cychc as far as chlorophyll and other cata- 

 lysts are concerned, the question whether a reaction takes place "before" 

 or "after" the primary photoprocess is not always as easy to answer as one 

 would at first imagine. We will assume that a dark reaction precedes the 

 photochemical step, if its retardation prevents the occurrence of this step 

 (and thus also all the succeeding ones), and that a dark reaction /o^/oius the 

 primary photochemical process, if the latter takes place in any case, and the 

 effect of the limited rate of the dark reaction is merely to cause an accumula- 

 tion of the primary photoproducts. Since experience shows that no large 

 accumulation of oxidation intermediates occurs in photosynthesis (this is 

 evidenced by the abrupt stoppage of oxygen production after the cessation 

 of illumination), we must assume that the primary oxidation products 

 ("photoperoxides") are unstable; unless rapidly removed or chemically 

 stabilized by a "finishing" process, they apparently disappear by back reac- 

 tions. 



The uptake of carbon dioxide may sometimes continue for about 20 sec. 

 in the dark {cf. Vol. I, page 200, and Vol. II, chapter 36). This may 

 mean that some intermediate reduction products survive for that length 

 of time, or that the carbon dioxide acceptor, A, requires it to become re- 

 carboxylated. (It may also be that the COa-acceptor is itself a reduction 

 intermediate of carbon dioxide cf. chapter 36.) 



The two alternative mechanisms of light saturation can thus be de- 

 scribed as starvation, which causes an "idling" of the primary photochemical 

 mechanism, and constipation, which blocks the elimination of the primary 

 products and compels most of them to return to their initial form. 



As described before, the distinction between the effects of prepara- 

 tory and finisliing dark reactions becomes still more diffi(.*ult, if we follow 

 Franck in the assumption that one of the finishing reactions "backfires," so 

 that its slowness, like that of the preparatory reactions, affects the composi- 



