Editing Signal transduction (section)

===Calcium===
The release of calcium ions from the [[endoplasmic reticulum]] into the [[cytosol]] results in its binding to signaling proteins that are then activated; it is then sequestered in the [[smooth endoplasmic reticulum]]<ref>{{Cite journal |display-authors=6 |vauthors=Wilson CH, Ali ES, Scrimgeour N, Martin AM, Hua J, Tallis GA, Rychkov GY, Barritt GJ |date=March 2015 |title=Steatosis inhibits liver cell store-operated Ca²⁺ entry and reduces ER Ca²⁺ through a protein kinase C-dependent mechanism |journal=The Biochemical Journal |volume=466 |issue=2 |pages=379–90 |doi=10.1042/bj20140881 |pmid=25422863}}</ref> and the [[mitochondria]]. Two combined receptor/ion channel proteins control the transport of calcium: the [[Inositol triphosphate receptor|InsP<sub>3</sub>-receptor]] that transports calcium upon interaction with [[inositol triphosphate]] on its cytosolic side; and the [[ryanodine receptor]] named after the [[alkaloid]] [[ryanodine]], similar to the InsP<sub>3</sub> receptor but having a [[feedback mechanism]] that releases more calcium upon binding with it. The nature of calcium in the cytosol means that it is active for only a very short time, meaning its free state concentration is very low and is mostly bound to organelle molecules like [[calreticulin]] when inactive.

Calcium is used in many processes including muscle contraction, neurotransmitter release from nerve endings, and [[cell migration]]. The three main pathways that lead to its activation are GPCR pathways, RTK pathways, and gated ion channels; it regulates proteins either directly or by binding to an enzyme.