Electrochemical differentiation between photosystem I and II in thylakoid membranes

Abstract

The current–voltage relationship of a thylakoid-containing electrochemical cell of small volume (80 μL) was determined in the dark and under illumination. The photocurrent produced was dependent on pH, on the type of artificial electron acceptor or inhibitor added, and on the electrode potential. Specific monitoring of photosystem-I and -II electron-transfer activity was possible. Photosystem II produced maximal photoeffect at pH 6.0, and the photocurrent–voltage behavior was monophasic. In contrast, photocurrent induction by photosystem I was maximal at pH 8.0 and a biphasic photocurrent–voltage dependency was obtained, presumably as a result of a redox potential barrier located on the donor side of photosystem I. It was concluded that the three-electrode system used is appropriate for measurement of the partial reactions of photosynthesis as well as the coupled photoreactions I and II using the same type of experiment.