FLASH YIELD IN HEAVY WATER 1465 



suggestion to account for the effect of narcotics on the rate of photosynthe- 

 sis in strong continuous Hght. 



Brilliant and Krupinikova (1952^) studied the effects of dehydration on 

 the yield of photosynthesis in constant and intermittent light. 



The observations of Arnold (1933) on the effect of ultraviolet light on 

 photosynthesis in flashing light offer a similar problem. As described in 

 chapter 13 (page 345), each quantum of ultraviolet light (253.6 m/x) ap- 

 pears to "knock out" one catalytic "center" (these centers are not chloro- 

 phyll molecules, since chlorophyll remains intact, but their concentration is 

 approximately equivalent to that of the green pigment) . Arnold found that 

 inhibition is proportionately the same in flashing and in continuous light, 

 and that the full yield per flash cannot be restored by extending the dark 

 intervals. The observation that the concentration of the sensitive cata- 

 lytic centers is about equal to that of chlorophyll makes one think of the 

 carbon dioxide acceptor, A, as the ultraviolet-sensitive target. If this is 

 so, the inhibiting effect of ultraviolet light on the flash yield must be 

 analogous to the effect of low carbon dioxide concentration. 



The effect on the flash yield of variations in [CO2], as well as that of the narcotics, 

 could be explained without much difficulty on the basis of the theory of "substrate limita- 

 tion" (c/. Arnold 1935) by assuming that at low [CO2I, or in the presence of narcotics, 

 only a fraction of the normal number of substrate molecules are available for immediate 

 reduction. 



Another possibility for solving this type of kinetic difficulty is by assum 

 ing a morphological or kinetic "photosjmthetic unit" (Gaffron and Wohl" 

 or "chlorophyll ensemble" (Tamiya) in which a number (say, 400-2000 

 of chlorophyll molecules are associated with a single "reduction center" or 

 a single molecule of an enzyme, in such a way that the intermediate photo- 

 products formed at these chlorophyll molecules can be further transformed 

 only in this one reduction center or by this one enzyme molecule, and react 

 back if they find this center or enzyme molecule occupied. (For a discus- 

 sion of such kinetic mechanisms, see, e. g., Rabinowitch 1951, and Gilmour 

 et al. 1953.) 



5. Flash Yield in Heavy Water 



Pratt and Trelease (1938) found that substitution of deuterium oxide for 

 ordinary water causes an extension of the Emerson- Arnold period to more 

 than twice the original value, without affecting the maximum yield per 

 flash (figure 34.20). The P = f{ta) curve is changed in a way reminiscent 

 of the effect of cyanide — it is linear almost to the point of saturation. 

 However, since the number of experimental points is small, this conclusion 

 is not quite certain. 



