In most cells, with the small exception of muscle and nerve cells when active, ion concentrations are in a steady state inside and outside the cell. Mechanisms of Passive Permeation Simple diffusion is allowed through aqueous channels in cell membranes. These channels are most likely proteins. They select what can pass by size differentiation. Diffusion of lipid soluble chemicals can occur across cell membranes as the membrane is primarily lipid in nature.
Some proteins act as carrier molecules that carry specific molecules across the membrane.
They carry molecules only down concentration gradients, do not use energy, and are specific in their receptors. Some channels within cell membranes have been found to be ion specific. That is, they allow ions to pass down their concentration gradient. They are ion specific by size, charge, and perhaps unexplained ways. Facilitated transport is the term given to transport across membranes by carrier molecules.