Calcium, as the ion Ca2+, is essential in many biological processes. The majority of Ca2+ in many organisms is bound to phosphates which form skeletal structures and also buffer Ca2+ levels in extracellular fluids (typically 1 millimolar). Intracellular free Ca2+, by contrast, is 10,000 times lower than the outside of the cell (typically 10 micromolar). This concentration gradient is used to import Ca2+ into cells where it acts as a second messenger.
De Smet, P,