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Photosynthetic reaction centers are integral membrane complexes that produce a net transmembrane charge separation in response to light. The Photosystem I (PSI) complex is a thoroughly studied reaction center that has been proposed as a nanoscale photovoltaic structure in diverse applications, including activation of excitable cells by triggering of voltage-gated ion channels. An electrostatic model of a spherical lipid vesicle embedded with PSI and suspended in an aqueous medium is presented. The distribution of the electric potential is obtained by solving the nonlinear Poisson-Boltzmann equation with the finite-element method. The model predicts a maximum potential difference of 1.3 V between charges. This value depends mostly on the intrinsic dielectric constants of the reaction center and distance between charges. However, the potential distribution near the reaction center depends on the ionic strength of the aqueous medium. When the ionic strength is zero, the vesicle develops a transmembrane potential that increases linearly with the density of reaction centers. When the ionic strength increases, this potential difference approaches to zero. The main results of the simulations are consistent with previously reported experimental data. Based on the presented results, the potential application of PSI to light activation of voltage-gated ion channels is discussed.