The Mg ion tends to bind only weakly to proteins (K ≤ 10) and this can be exploited by the cell to switch enzymatic activity on and off by changes in the local concentration of Mg. Although the concentration of free cytoplasmic Mg is on the order of 1 mmol/L, the total Mg content of animal cells is 30 mmol/L and in plants the content of leaf endodermal cells has been measured at values as high as 100 mmol/L (Stelzer et al., 1990), much of which buffered in storage compartments. The cytoplasmic concentration of free Mg is buffered by binding to chelators (e.g., ATP), but also, what is more important, it is buffered by storage of Mg in intracellular compartments. The transport of Mg between intracellular compartments may be a major part of regulating enzyme activity. The interaction of Mg with proteins must also be considered for the transport of the ion across biological membranes.