The Mechanism of the Photochemical Activity of Isolated Chloroplasts. III. Dependence of Velocity on Light Intensity.

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Abstract
The experimental steady state velocity-light relationship for the Hill reaction at low oxidant concentrations is found to be a rectangular hyperbolic function within narrow statistical limits. The same form of the rate-law is shown to apply at high oxidant concentrations where there appears to be extensive decoupling of photophosphorylation from Hill reaction. The rate law is analyzed in terms of the microscopic rate law which pertains in the region of a few hundred chlorophyll molecules to provide a means for evaluating the fundamental rate parameters. The experimental rate is shown to depend on the average light intensity through the reaction cell and thus reveals that apparent steady-state experiments are actually flashing-light studies. The activation energy for the velocity-controlling elementary process at low light is shown to be less than 0.1 k. cal. whereas that for the limiting step at high intensity is 13.0 + 0.3 k. cal. (Swiss chard, rhubarb chard). The quantum requirement was found to be 35% higher at 675m [mu] than at 560 and 457.5 mu.