KINETICS 1603 



10° C; the loss at 25° C. was particularly high in the absence of chloride 

 (when the photochemical deactivation of the chloroplasts is particularly- 

 fast). 



Gilmour et at. (1953) found a linear relation between log P™'*''- and 1/T 

 with ferri cyanide as oxidant, for the range 6-23° C. ; the slope corresponded 

 to an activation energy of 7.4 kcal/mole. They reported that Bishop 

 (1952) had observed, with the chloroplasts from the same species {Beta 

 vulgaris) an activation energy of 10 kcal/mole. 



Concentration of Chloroplasts. Variations in the concentration of 

 chloroplasts in the suspension could influence the rate of the Hill reaction 

 in two ways. One is trivial: with increased optical density the light ab- 

 sorption first increases almost proportionally with concentration, then 

 rises more slowly, and finally approaches totality. The average absorp- 

 tion per chlorophyll molecule is at first almost constant and then decreases 

 steadily. The same must be true of the yield per unit chlorophyll amount 

 in the lower part of the light curve, where light supply is the rate-limiting 

 process. In saturating light, the rate per unit chlorophyll amount should 

 be independent of chloroplast concentration ; but the higher this concentra- 

 tion, the more light will be needed to reach this saturation. (These rela- 

 tions have been discussed before, in chapters 25, 28 and 32.) 



A second, more significant concentration effect could be caused by 

 ''poisoning" of the medium by products of respiration or fermentation of 

 the chloroplast fragments; this inhibition (if it occurs at all) should be 

 strongest in the most concentrated suspensions. 



Observations on the rate of Hill reaction in suspension of different con- 

 centrations so far fall into the first, trivial category. 



Spikes et al. (1950) found a proportional increase in the rate of ferricy- 

 anide reduction by spinach chloroplasts in a 0.25-cm. thick vessel at [Chi] 

 values from 5 to 40 X 10^^ mole/liter in red light of "5000 lux" (this 

 must mean intensity of white light before passing through a red filter, and 

 seems to be too low a light for saturation, particularly in a concentrated 

 suspension). Holt and French (1946) made the same observation in a 

 much stronger light (4500 foot candles, or about 45,000 lux of white light 

 before passing through a red filter) in a vessel about 0.5 cm. thick, and at 

 chlorophyll concentrations up to but not in excess of 16 X 10 ~^ mole/liter. 



Clendenning and Gorham (1950^) found the rate of the reaction with 

 Hill's mixture to be proportional to [Chi] up to 7.5 X 10"^ mole/liter. 



Wessels (1954) considered a different effect of chloroplast concentration 

 — that on the final "photostationary" state (rather than on the rate of ap- 

 proach to this state). Figure 35.23A illustrates the finding that while the 

 initial rate of reduction of DCPI increases, as expected, with the concen- 

 tration of chloroplasts, the photostationary state (as measured by the 



