Active transport mechanisms can accumulate compounds against a concentration gradient, so that it may be much higher on one side of the membrane than the other. This concentration requires an expenditure of energy. There are two primary mechanisms of active transport, each utilizing a different form of energy.
Transport Systems that Use Proton Motive Force Many bacterial transport systems can accumulate or extrude small molecules and ions using the energy of a proton motive force. Transporters of this type allow a proton into the cell and simultaneously either bring along or expel another substance (figure 3.28). For example, the permease that transports lactose brings the sugar into the cell along with a proton. Expulsion of waste products, on the other hand, relies on transporters that eject the compound as a proton passes in. Efflux pumps, which are used by some bacteria to oust antimicrobial drugs, use this latter mechanism. These systems are part of a large group of transporters, collectively known as the major facilitator superfamily (MFS), found in prokaryotes as well as eukaryotes.
Transport Systems that Use ATP ABC transport systems require ATP as an energy source (ABC stands for ATP Binding-Cassette). These systems are relatively elaborate, involving multiple protein components (figure 3.29). The ABC transport system uses a binding protein that resides immediately outside of the cyto-plasmic membrane to deliver a given molecule to a specific transport complex within the membrane. Numerous different binding proteins exist for the various sugars and amino acids. Each binds tightly to its respective molecule and, therefore, the binding protein can effectively scavenge even low concentrations of a molecule and bring them into the cell. The sugar maltose is an example of a molecule that is transported by an ABC transport system.
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