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. The mechanism involves TonB protein, which, in a novel mechanism, appears to shuttle between the membranes in the process of transducing the energy signal. 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. These include not only the TonB-dependent systems found in all Gram negative bacteria, but also systems for secretion of various proteins and toxins into the cellular environment. Furthermore, the ability of bacterial pathogens to obtain iron is a TonB-dependent process.