1900 



KINETICS OF PHOTOSYNTHESIS 



CHAP. 37D 



intermediates as substrates of photochemical "antirespiration" are not 

 prevented by cyanide from doing so, since cyanide affects primarily one 

 of the reactions through which carbon dioxide enters the photosynthetic 

 cycle. (This is indicated by the several observations quoted on pp. 

 307-308, and by the indifference to cyanide of the Hill reaction.) 



Whether the cyanide-sensitive reaction is the primary carboxylation 

 itself, as suggested in chapter 12, or a follow-up reaction of the ACO2 

 complex (as suggested by Tamiya and others) is as yet uncertain. (The 



.3 .4 .5 



pCOj in Xatm. 



Fig. 37D.8. Carbon dioxide curves of Scenedesmus in the absence and presence of 

 cyanide, in 2 X 10 ~^ M bicarbonate (determined with pB. meter) (after Gaffron 1953, 

 Rosenberg 1954). Suspension density, 0.5 volume percent. (Not corrected for changes 

 in buffer caused by KCN.) Curves indicate that the [CO2] -dependence is not a diffusion 

 effect. 



lack of sensitivity to cyanide of known carboxylases — except the "hydro- 

 gen lyase," which combines carbon dioxide and molecular hydrogen to 

 formic acid — is an argument against the first hypothesis.) 



Fluoride. Simonis (1949) investigated the effect of sodium fluoride 

 on photosynthesis. (This agent is of interest because it inhibits trans- 

 phosphorylation reactions.) 



Previously, French (1946) had mentioned that the Hill reaction is 

 sensitive to fluoride; but Warburg (1947) found no effect of fluoride on 

 Hill reaction with quinone as oxidant. Simonis spread excised leaflets of 

 Mnium undulatum on a metal net, covered them with fluoride solution and 

 illuminated with white light (1.3, or 13 klux). Photosynthesis was deter- 

 mined manometrically. At 13 klux, one hour incubation (at 17° C.) 



