Freely exchangeable magnesium

What do /oosriy bound and freely exchangeable magnesium mean "The reversible interaction between two metabolites, such as a metal ion (M) and a protein (P), can be expressed as... 

The total concentration of Mn in the liver cell is about 35 iM, about 1/1000th that of magnesium. Free Mn occurs at about 0.2 to 1.0 iM in the liver cell. Loosely bound and freely exchangeable Mn occur at about 12. iM, with the rest of the ion occurring tightly bound to proteins. Mitochondria contain a concentration of Mn equivalent to 300 iM, about 1/100th that of mitochondrial magnesium (Senior et al, 1980). 

Exchange among any of the four compartments is not limited to calcium and phosphate ions but may include any other species present in the bulk solution. Depending on the ion, it will be more or less concentrated in the hydration shell and will, or will not, be able to exchange with ions in the crystal lattice. Monovalent ions such as Na, F and K enter the hydration shell but do not become concentrated within it. Equilibrium is established between the ions in the bulk solution and those in the hydration shell. The polarizable ions include hydrated ions, especially multivalent ones, such as calcium, carbonate, citrate, magnesium and strontium, which, because of their ability to neutralize the surface charge on the crystals, tend to concentrate in the hydration shell where they are known as the bound ion layer. These ions are freely mobile, however, and can exchange readily with ions in the bulk solution. 

---