However, membranes are not only passive barriers. They include a set of proteins specialized for promoting or catalyzing various cellular processes (Nelson & Cox 2004, p.369). Molecular transportation is the most important function of membranes facilitated by carrier and channel proteins (McKee 2004, p.62).
General questions of transportation and channeling across biologic membranes are well considered in the encompassing textbooks of Lodish (2003, p.245-300), Nelson & Cox (2004, p.369-420), Kuchel & Ralston (1997, p.171-184), McKee (2004, p.353-366), Garrett & Grisham (1999, p.259-326), etc., in special monographs, e.g. Keizer (2000) and also in a variety of articles.
All cells acquire from its environs the raw materials for biosynthesis and for energy production, and also release to its environment the byproducts of metabolism. Only some nonpolar compounds can cross the membrane unassisted. However, for polar or charged compounds or ions, a membrane protein is essential for transmembrane movement.
Membrane transport mechanisms are vital to living organisms. Ions and molecules constantly move across cell plasma membranes and across the membranes of organelles. This flux must be regulated to meet each cell's metabolic needs. For example, a cell's plasma membrane regulates the entrance of nutrient molecules and the exit of waste products. Additionally, it regulates intracellular ion concentrations. Because lipid bilayers of membranes are generally impenetrable to ions and polar substances, specific transport components must be inserted into cellular membranes (McKee 2004, p.372).
In the simplest cases a membrane protein facilitates the diffusion of a solute down its concentration gradient. However, transportation often occurs against a gradient of concentration, electrical charge, or both. In such cases, solutes must be "pumped" that requires energy. The necessary energy may come from ATP hydrolysis (i.e. directly), or may be supplied indirectly, e.g. in the form of movement of another solute down its electrochemical gradient with enough energy to carry another solute up its gradient (Nelson & Cox 2004, p.389; Garrett & Grisham 1999, p.296).
Ions may also move across membranes via ion channels formed by proteins, or they may be carried across by ionophores, small molecules that mask the charge of the ions and allow them to diffuse through the lipid bilayer of membrane. With very few exceptions, the traffic of small molecules across the plasma membrane is mediated by proteins such as transmembrane channels, carriers, or pumps (Nelson & Cox 2004, p.391).
So, ions cannot pass freely through the cell's phospholipid membrane. Instead, most ions flow through special channels built from multiple protein subunits. These subunits together form a pore across the membrane. Some channels are gated, fitted with proteins that act "as hinged doors, blocking the opening until