Most intracellular magnesium is complexed with ATP or other negatively charged molecules. In contrast, their intracellular levels are vastly different from each other and these ions perform very different functions. Alterations in plasma levels of either one of these species have major effects on the behavior of excitable cells because they alter the electric field sensed by voltage-gated channels. However, being divalent, they are major contributors to the layer of cations that are attracted to negative charges on plasma membranes and large plasma proteins. Their free (non-complexed) concentrations in the ECF are roughly the same (∼1 mM), low enough so that they are not significant components of plasma osmolality.
Calcium and magnesium are both divalent alkali metal cations that play similar roles in some contexts and different roles in others. The physiology and regulation of calcium, magnesium, and phosphate are critically dependent on their chemical properties.
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