<P> Calcium ions (Ca) play a vital role in the physiology and biochemistry of organisms and the cell . They play an important role in signal transduction pathways, where they act as a second messenger, in neurotransmitter release from neurons, in contraction of all muscle cell types, and in fertilization . Many enzymes require calcium ions as a cofactor, those of the blood - clotting cascade being notable examples . Extracellular calcium is also important for maintaining the potential difference across excitable cell membranes, as well as proper bone formation . </P> <P> Calcium levels in mammals are tightly regulated, with bone acting as the major mineral storage site . Calcium ions, Ca, are released from bone into the bloodstream under controlled conditions . Calcium is transported through the bloodstream as dissolved ions or bound to proteins such as serum albumin . Parathyroid hormone secreted by the parathyroid gland regulates the resorption of Ca from bone, reabsorption in the kidney back into circulation, and increases in the activation of vitamin D to calcitriol . Calcitriol, the active form of vitamin D, promotes absorption of calcium from the intestines and the mobilization of calcium ions from bone matrix . Calcitonin secreted from the parafollicular cells of the thyroid gland also affects calcium levels by opposing parathyroid hormone; however, its physiological significance in humans is dubious . </P> <P> Calcium storages are intracellular organelles, that constantly accumulate Ca ions and release them during certain cellular events . Intracellular Ca storages include mitochondria and the endoplasmic reticulum . Characteristic concentrations of calcium in model organisms are: in E. coli 3mM (bound), 100nM (free), in budding yeast 2mM (bound), in mammalian cell 10 - 100nM (free) and in blood plasma 2mM . </P> <P> In vertebrates, calcium ions, like many other ions, are of such vital importance to many physiological processes that its concentration is maintained within specific limits to ensure adequate homeostasis . This is evidenced by human plasma calcium, which is one of the most closely regulated physiological variables in the human body . Normal plasma levels vary between 1 and 2% over any given time . Approximately half of all ionized calcium circulates in its unbound form, with the other half being complexed with plasma proteins such as albumin, as well as anions including bicarbonate, citrate, phosphate, and sulfate . </P>

Where are calcium ions stored in a cell