Electrogenesis in the photosynthetic membrane: fields, facts and features

This review deals with electrical events that occur in the photosynthetic (thylakoid) membrane of chloroplasts after light energy capture in the photochemically active reaction centers of the two photosystems of photosynthesis. A simplified electrical model of the chloroplast as a light-driven energy transducer is given. Principles and methods for measuring the generation and decay of electric fields in the membrane and of transmembrane potentials and currents are described. Special emphasis is given to prospects and limitations of an optical method, based on electrochromicity of endogeneous pigments (P515), and of electrophysiological methods using glass microelectrodes for either impaling or patch-clamping a single chloroplast. It is illustrated that application of both methods enables one to (i) estimate the number of energy conserving competent (active) photosystems, (ii) measure the magnitude of the potential generated by a single turnover of the photosystems, (iii) estimate the kinetics and the number of charges involved in secondary electrogenic transport across the thylakoid membrane, (iv) study the conductance behavior of the thylakoid membrane, and (v) study the energy-dependent conductance changes originating from the lumenal and stromal phases adjacent to the thylakoid membrane. (C) 1997 Elsevier Science S.A.