13 The Free-Energy Transduction and Entropy Production in Initial Photosynthetic Reaction

Abstract

Initial photosynthetic reactions in bacterial photosynthesis are modeled in this chapter as transitions among discrete states of integral membrane proteins performing these reactions. The steady state entropy production is then associated with each transition starting from photon absorption. The assumption of maximum entropy production in all irreversible non-slip transitions leads to high free energy transfer efficiency (high quantum yield) and to optimal values of kinetic constants that are comparable to experimentally determined values. Optimal overall efficiency of close to 20% is similar to measured values for the efficiency of producing the protonmotive power in reconstituted systems. We conclude that photosynthetic proton pumps operate close to maximum entropy production regime and use the advantage of nonlinear flux-force relationships to transfer power with around 90% efficiency instead of 50% as prescribed by the maximal power transfer theorem in the linear regime. Finally, the evolution-coupling hypothesis is suggested, according to which photoconverters couple their own evolution to thermodynamic evolution in a positive feedback loop accelerating both evolutions.