Give the biological importance of calcium and magnesium.

(N/A) An adult body contains about $25 \ g$ of $Mg$ and $1200 \ g$ of $Ca$,compared with only $5 \ g$ of iron and $0.06 \ g$ of copper. The daily requirement in the human body is estimated to be $200-300 \ mg$.
All enzymes that utilize $ATP$ in phosphate transfer require magnesium as a cofactor. The main pigment for the absorption of light in plants is chlorophyll,which contains magnesium. About $99 \%$ of body calcium is present in bones and teeth. It also plays important roles in neuromuscular function,interneuronal transmission,cell membrane integrity,and blood coagulation.
The calcium concentration in plasma is regulated at about $100 \ mg \ L^{-1}$. It is maintained by two hormones: calcitonin and parathyroid hormone.
Bone is not an inert and unchanging substance but is continuously being solubilized and redeposited to the extent of $400 \ mg$ per day in humans. All this calcium passes through the plasma.