Calcium and Life

Calcium is the fifth most abundant element in the biosphere, after oxygen, silicon, aluminum, and iron. It is present in high concentration in seawater and in all fresh waters that support an abundant biota. Fortuitously, the calcium ion has just the right radius to fit neatly within the folds of various peptide chains. Calcium thereby stabilizes and activates a large number of structural and catalytic proteins essential for life. In this capacity calcium serves as a ubiquitous second messenger within cells, mediating such diverse processes as mitosis, muscle contraction, glandular secretion, blood coagulation, and interneuronal signal transmission. Controlling these activities requires careful regulation of the concentration of calcium in critical fluid compartments. This regulation is accomplished in two basic ways.

At a cellular level, calcium is ordinarily sequestered within intracellular storage compartments. It is released into the cell sap when needed to trigger various cellular activities, and then quickly pumped back into its storage reservoirs when the activity needs to be terminated. This control mode is exemplified by the accumulation and release of calcium by the sarcoplasmic reticulum of striated muscle. The second type of control, utilized by many tissues in higher organisms, is the tight regulation of the calcium level in the blood and extracellular fluids that bathe all the tissues. Individual cells, needing a pulse of calcium, simply open membrane channels and let calcium pour in from the bathing fluid; they then pump it back out when the particular activity needs to cease.