Simulation of the electrostatic field (shown in yellow) for one of the four-sided channels at the outer surface of ferritin shows that the inner walls of the channel are immersed in a region of positive electrostatic potential, and the region surrounding the outer entrance to the channel is very negative. The electrostatic field generated by this charge distribution should pull a positive particle from the channel interior toward the exterior of the protein cavity. On the interior of the molecule, entrances to the same channels are surrounded by strong negative charges, suggesting a mechanism for attracting positive particles (perhaps protons) into the channel for removal.