The ability to regulate transport across cellular boundaries is essential to the cell’s existence as an open system. There is a steady traffic of ions, molecules, polymers, and other species across the plasma membrane. Consider the chemical exchanges between a human muscle cell and the extracellular fluid that surrounds it. Sugars, amino acids, and other nutrients enter the cell; waste products of metabolism leave. The cell takes in oxygen for cellular respiration and expels carbon dioxide. It also regulates its concentrations of inorganic ions, such as Na+, K+, Ca2+, and Cl−, by shuttling them one way or the other across the plasma membrane. Mother Nature has created natural channels that are highly selective, that is, they allow certain molecules and ions to pass more easily than others (or they reject them). For example, there are highly specialized potassium protein channels that allow potassium cations to pass through with high selectivity than other ions. Other examples include water, Na+, Ca2+, Cl−, glucose protein channels, and nuclear pore complexes. The highly selective transportation of molecules and ions is controlled by molecular recognition between transporting species and protein channels present in the cell membranes.
