Publisher Summary This chapter discusses that study of the transport of substrates into intact bacteria is often complicated (1) by multiple sources of energy by multiple transport pathways, (2) by difficulties in characterizing the intracellular milieu, and (3) by catabolic processes that decrease the size of metabolite pools. For these reasons, envelope vesicles lacking most cytoplasmic components have been prepared from a variety of bacteria. The ease of manipulating the internal contents of such vesicles is a particularly valuable property, because transmembrane ionic gradients are recognized to play a major role in coupling metabolic energy to the accumulation of various substrates. Membranes of halobacteria contain a relatively simple H+ translocation system, consisting of the protein–retinal complex called “bacteriorhodopsin,” which is activated by light. Bacteriorhodopsin molecules are located in distinct “purple membrane” patches, continuous with the cytoplasmic membrane. Transport in these vesicles can be energized either by such H+ movements or by prearranged cation gradients.