Figure 7. Operation of bacterial transport systems. Bacterial transport systems are operated by transport proteins (sometimes called carriers, porters or permeases) in the plasma membrane. Facilitated diffusion is a carrier-mediated system that does not require energy and does not concentrate solutes against a gradient. Active transport systems such as Ion-driven transport and Binding protein-dependent transport, use energy and concentrate molecules against a concentration gradient. Group translocation systems, such as the phosphotransferase (pts) system in Escherichia coli, use energy during transport and modify the solute during its passage across the membrane.