ON THE INTERACTION OF PHOTOSYSTEMS I AND II IN ALGAL CELLS*,†

Abstract— Steady-state relaxation spectrophotometry was applied to intact algal cells. The results indicated System I operates in a cyclic manner, only indirectly affected by the presence or absence of oxygen evolution. This action in whole cells is distinct from that occurring in chloroplasts, where System I appears tightly coupled to System II. No evidence was found for the oxidation of cytochrome by P700. Fluorescence emission measurements on intact cells emphasized the role of membrane permeability agents in controlling the emission yield. Large changes in the yield caused by Systems I and II could be observed in the presence of these agents. In sum, the results proved complex and were difficult to resolve in any simple scheme but emphasized the possible role of protons (or membrane potential) in controlling fluorescence yield.