STUDIES ON PHOTOPHOSPHORYLATION: I. TWO-STEP EXCITATION KINETICS OF PHOTOPHOSPHORYLATION1

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Abstract
Based on the observations on the time lag and the low efficiency of photophosphorylation at low light intensities, a ‘two-step excitation kinetics’ of photophosphorylation of chloroplasts was developed, which could explain some of the hitherto unexplained facts concerning photophosphorylation. By analyses of the experimental results along the line of the theory, it was inferred that the assumed substance (X) responsible for ATP formation occurs in a relatively large quantity of about 1 mole/5–20 moles of chlorophyll. It was concluded that the high energy intermediate (X**) of ATP formation which was assumed to accumulate on pre-illuminating the chloroplasts (in the absence of ADP and Pi) does not represent a side pool of extra energy, but a substantial intermediate in the normal course of ATP formation. Alternative possibilities for the explanation of the experimental facts were also discussed.