PHYSIOLOGICAL STUDIES 7I 



KINETIC FORMULATIONS OF PHOTOSYNTHESIS BASED ON 

 RESPONSES OF THE PLANT 



The slow development of activity in the study of the bio- 

 chemistry of photosynthesis meant that precise ideas as to 

 the chemical mechanism of the process involved could not 

 at first be formulated. From the observed physiological 

 responses of the plant, physiologists suggested a possible 

 mechanism of the process in terms of the minimum number 

 of arbitrary postulates. 



The simplest type of mechanism postulated that photo- 

 synthesis was a direct 'photolysis' of carbon dioxide 

 catalysed by chlorophyll. But as we have seen in this chapter 

 a one-step process is too simple to provide an interpreta- 

 tion of the effect of external factors on the rate of the over- 

 all process. Willstatter and StoU formulated a mechanism to 

 take account of some of these effects. They suggested a 

 three-step process involving (i) a reaction between carbon 

 dioxide and chlorophyll to form a chemical complex, (2) 

 the absorption of light by this complex resulting in the 

 formation of a peroxide, and (3) a dark reaction in which 

 oxygen is evolved from the peroxide with the liberation of 

 free formaldehyde. The third reaction was regarded as an 



^ light / 



Chl+H,C03 - Chl.HOC ^ Chl.HOCH 



Chl+H.CHO+Oa 



Chi = chlorophyll 



enzymic process involving an 'internal factor' of a type 

 which had been postulated with reference to work on the 

 assimilation number, and which has been discussed earlier 

 in this chapter. 



A slightly different mechanism was first proposed by 

 Warburg (1919), later abandoned by him in 1924, but 

 recently more fully developed by Briggs, in 1935 and 1941. 

 In Briggs' formulation the chemical nature of the reactants 

 is not specified. Essentially there are three reactions, (i) 

 the formation of a complex between carbon dioxide and 



