6 - PHOTOSYNTHETIC GENERATORS OF PROTONMOTIVE FORCE

Abstract

This chapter describes the photosynthetic generators of proton-motive force. A photosynthetic organism captures light energy to drive the otherwise endergonic synthesis of molecules needed for the growth and maintenance of the organism. A central feature of photosynthesis is the conversion of light energy into redox energy, meaning that photon capture causes a component to change its redox potential from being relatively electropositive to being highly electronegative. The electrons released from this component are utilized to generate a proton-motive force, flowing either through a cyclic pathway back to re-reduce the original component or in a noncyclic pathway to reduce additional electron acceptors. In this latter case, a continual electron supply to the photon-sensitive component is required. The two features that are unique to photosynthetic systems are the antennae, responsible for the trapping of photons, and the reaction centers, to which the light energy is directed. A component in the reaction centre becomes electronically excited as a result of the absorption of a photon. An electron can be released from this excited state at a potential that is up to 1 V more negative than the potential of donors to the reaction centre. Thus, the electron lost from the reaction centre is replaced by an electron at much more positive potential so as to regenerate the ground state of the component in the reaction centre that underwent excitation. In this way, light energy is directly transduced into redox potential energy.