Biological Importance of Sodium, Potassium, Magnesium and Calcium

Four s-block elements -- sodium, potassium, magnesium and calcium -- are essential (biologically indispensable) elements, each with a distinct, well-established physiological role.

Sodium and potassium. These two ions together maintain the electrolyte and osmotic balance of body fluids and are central to the transmission of nerve impulses. $\text{Na}^+$ is the principal cation of the fluid outside cells (the extracellular fluid), while $\text{K}^+$ is the principal cation inside cells (the intracellular fluid); this sharply uneven distribution is actively maintained, against the natural tendency of ions to diffuse and equalize, by the membrane-bound enzyme $\text{Na}^+$/$\text{K}^+$-ATPase (the sodium-potassium pump), which uses the energy of ATP hydrolysis to pump three $\text{Na}^+$ ions out of the cell for every two $\text{K}^+$ ions it pumps in. This carefully maintained concentration gradient across the nerve cell membrane is exactly what a nerve impulse discharges and restores: a stimulus briefly opens $\text{Na}^+$ channels in the membrane, letting $\text{Na}^+$ rush into the cell and reverse the local membrane potential (depolarization), producing the electrical signal called the action potential; $\text{K}^+$ channels then open to let $\text{K}^+$ flow back out, restoring (repolarizing) the resting potential, ready for the pump to re-establish the original ion gradients. The same $\text{Na}^+$/$\text{K}^+$ balance is also essential for regulating the water content and osmotic pressure of body fluids.

Magnesium. $\text{Mg}^{2+}$ sits at the very centre of the porphyrin ring of chlorophyll, the green pigment in plants that captures light energy for photosynthesis -- exactly the role the iron ion plays at the centre of the haem ring in haemoglobin. Without this magnesium ion coordinated at the core of the ring, chlorophyll cannot function, making magnesium indispensable to plant life and, by extension, to essentially the entire food chain that depends on photosynthesis. Magnesium ions are also required as a cofactor by a great many enzymes, particularly those that catalyse reactions involving ATP, and a substantial fraction of the body's magnesium is also stored in bone. …