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Calcium in biology
Calcium ions (Ca2+) play a pivotal 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.
Calcium levels in mammals are tightly regulated,〔 electronic-book ISBN 978-94-007-5561-1 electronic-
〕 with bone acting as the major mineral storage site. Calcium ions, Ca2+, 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 Ca2+ from bone, reabsorption in the kidney back into circulation, and increases in the activation of vitamin D3 to Calcitriol. Calcitriol, the active form of vitamin D3, 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.
Calcium storages are intracellular organelles, that constantly accumulate Ca2+ ions and release them during certain cellular events. Intracellular Ca2+ storages include mitochondria and the endoplasmic reticulum.〔Wilson, C.H., Ali, E.S., Scrimgeour, N., Martin, A.M., Hua, J., Tallis, G.A., Rychkov, G.Y., and Barritt, G.J. (2015). Steatosis inhibits liver cell store-operated Ca(2)(+) entry and reduces ER Ca(2)(+) through a protein kinase C-dependent mechanism. Biochem J 466, 379-390. http://www.ncbi.nlm.nih.gov/pubmed/?term=Eunus+S+Ali〕
==Eukaryota==
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