Illustration of a model for the potassium and sodium ion channels through pores in the plasma membrane of an animal nerve cell: an example of active transport, where solutes (ions, sugars, amino acids) are pumped into cells from a region of lower to higher concentration. This is a reverse of free diffusion, where molecules move from higher to lower concentrations, that is, down the concentration gradient. The sodium/potassium exchange pump is described here: sodium ions (Na+, red spheres) are being actively pumped out of the cell in a compulsory exchange for potassium ions (K+, blue spheres) entering the cell.