The size of the function unit of electrical events in thylakoid membranes was estimated by the minimum amount of gramicidin needed to discharge the flash light generated electrical potential difference. Early flash spectroscopic measurements have indicated that a single gramicidin dimer operates on an electrical function unit containing at least 2 × 105 chlorophyll molecules [1]. In this study we present gramicidin titrations with more intact thylakoid preparations which revealed a more than hundred-fold greater lower limit for the electric unit size, namely 5 × 107 chlorophyll molecules. It is conceivable that the whole complicated thylakoid structure inside a chloroplast constitutes a single electric unit. It comprises more than 2 × 108 chlorophyll molecules in an area of more than 400 μm2.
