Calcium ions are involved in regulation of many biological systems. Such a regulatory function requires compartmentalization and structures capable of extracting Ca2+ from and, alternatively, delivering Ca2+ to discrete domains. Generally, this function is sustained by biological membranes with a variety of calcium transport systems. For instance, it is well established that the Ca2+ concentration in the interior of cells is three or four orders of magnitude lower than the Ca2+ concentration in the extracellular environment. The related Ca2+ gradient is maintained by transport systems located in the plasma membrane. Furthermore, in the interior of cells, it is possible to find membranes that are capable of concentrating Ca2+ into intracellular compartments.
