32] Energization of solute transport in membrane vesicles from anaerobically grown bacteria

Abstract

In bacteria, the cytoplasmic membrane is the only layer of the cell envelope that forms a diffusion barrier for most solutes. Solutes can pass the cytoplasmic membrane by systems that do not require metabolic energy—namely, diffusion or facilitated diffusion or by systems that depend on metabolic energy such as group-translocation and active transport. In group translocation, the transported solute molecule is chemically modified in such a way that the reaction itself results in the passage of the molecule through the cytoplasmic membrane. In active transport, the solute is translocated in unmodified form by a specific carrier protein against an electrochemical or osmotic gradient. The orientation of the vesicle membrane is the same as that of the membrane in intact cells. Membrane vesicles do not perform active transport of solutes unless energy is supplied in the proper form. In membrane vesicles from aerobically grown bacteria, energy for active transport processes can be supplied by electron flow in the respiratory chain to oxygen as terminal electron acceptor. In membrane vesicles from anaerobically grown bacteria, energization of active transport under anaerobic conditions can occur by electron transfer in anaerobic electron transfer systems or, in membrane vesicles from phototrophically grown bacteria, by light-induced electron transfer in cyclic electron transfer systems.