These particular antibiotics are not involved in the normal passive transport of ions by cells, but they are believed to be models for real carriers. Glycerol is brought into red blood cells, and galactose into E. coli bacteria, by carriers known as permeases. These permeases are thought to be enzymes, although little else is known about them.

All such permeases and carrier molecules are still only aids to the movement of molecules "downhill" along a concentration gradient. A more useful talent is the ability to carry ions or molecules from regions where they are scarce to regions where they are already concentrated, and to build up an excess on one side of the membrane. K and Na ions, phosphate, sugars, and some amino acids are concentrated by this active-transport process, which provides a way of gathering nutrients and storing them inside the cell for later use.

Energy is required to bring things in or out against a concentration gradient, and this energy is supplied by ATP.