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Biological role of Sodium, Potassium, Magnesium and Calcium

It has been found that an average human body weighing 70 kg contains 0.07 kg of sodium, 0.25 kg of potassium, 0.042 kg of magnesium, 1.7 kg of calcium and the rest, other elements. Sodium ions and potassium ions play different roles in metabolism, Na+ being an extra cellular ion and K+, an intracellular ion. Size of ions and types of bonding characteristics of a given ion are the two factors that determine which ions can be substituted by others in metabolic processes. The K+ ions may be substituted in living tissues by large single-charged Rb+, Cs+, NH4+and Tl+ ions while the relatively small Na+ ions can be substituted by Li+ ions. The mammalian tissues incorporate a sodium or potassium ion transport system called sodium (or potassium) pump. The sodium pump ensures the desired ratio of Na+ and K+ concentrations in the extra- and intracellular spaces. The K+ ion concentrations in most animal cells ranges from 0.12 to 0.16 mol dm-3, while that of Na+ ion in the same cells does not exceed 0.01 mol dm-3. The ratio in the extracellular fluid is reversed, i.e. Na+ ion concentration 0.15 mol dm-3 and that of K+ ion 0.004 mol dm-3. Thus, there is a considerable gradient of concentrations of Na+ and K+ ions between the two cellular spaces. To maintain the distribution, energy is needed for K+ ions to accumulate inside cells and Na+ ions to be taken out. It has been confirmed that the pump driving K+ ions into cells and forcing Na+ ions out, functions aided by phosphate protein which forms more stable compounds with K+, as compared to Na+. The K+ ion traverses the cell membrane as a constituent of a complex with phosphate protein. Once inside the cell, the phosphate protein interacts with adenosine phosphate. The new ligand forms a more stable complex with Na+ ions, as compared to K+, and they are removed from the cell into the extracellular space. Magnesium and calcium perform vital functions in biological system. Green plants conduct the process of photosynthesis in the presence of sunlight and the magnesium containing pigment chlorophyll.

Large amounts of calcium are present in the bone tissue. The approximate composition of bone crystals corresponds to Ca10(PO4)6 (OH)2. Calcium is involved in enzymatic systems. It plays a role in regulating muscle contraction, transmitting nervous pulses and acts as an agent of blood coagulation.

Structure of chlorophyll

Calcium enters the body with food in the form of neutral phosphate which is converted into the readily soluble acid phosphates, CaHPO4 and Ca(H2PO4)2, by the acidity in the digestive tract. These acid phosphates are absorbed in the intestine and penetrate the blood plasma. The concentration of Ca2+ ions in human blood ranges between 0.0022 and 0.0028 mol dm-3. Nearly half of this calcium is in the form of aquo-ions capable of permeating membranes while the rest is bound with albumin and does not pass across membranes. Along with K+ and Mg2+, the Ca2+ ions affect the rate of muscular contraction including that of cardiac muscle, and the action of cardiac glycosides. An overdose of glycosides leads to cardiac arrest. Injection of K+ and Mg2+ ions into the cardiac muscle mitigates the action of glycosides, while that of Ca2+ enhances it. Excess calcium present in the body leads to the formation of stones, deposition of salts, etc.

The Mg2+ and Ca2+ ions do not substitute for each other in biosystems because of pronounced covalency of the bond between Mg2+, as opposed to Ca2+, and ligands.

Thus, we find that Na+, K+, Mg2+ and Ca2+ ions perform a variety of functions in animal and plant organisms. Many of the processes involving these ions are quite complex and are yet to be completely understood.

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