The cytoplasmic membrane is said to be energized, while the outer membrane is not.
Most nutrient molecules can traverse the outer membrane without requiring energy.
The source of energy for transport of those molecules across the outer membrane is, surprisingly, the cytoplasmic membrane. TonB is thus most analogous to a signal transducer, and in this case the signal being transduced is energy. The ability of bacterial pathogens to obtain iron is a TonB-dependent process. Since pathogens lacking TonB are compromised in their ability to cause disease, understanding the mechanism of TonB-dependent energy transduction could lead to the development of novel chemotherapeutics. Furthermore, over one hundred systems are known in Gram negative bacteria where proteins in the cytoplasmic membrane are proposed to contact proteins in the outer membrane.