Q.Explain the significance of sodium, potassium, magnesium and calcium in biological fluids.

Sodium (Na$^+$)

Sodium is the principal cation of extracellular fluid (blood plasma, interstitial fluid). It helps regulate the body's water/osmotic balance and blood pressure/volume, and, working with K$^+$ via the Na$^+$/K$^+$ pump across cell membranes, is essential for the transmission of nerve impulses and for muscle contraction.

Potassium (K$^+$)

Potassium is the principal cation of intracellular fluid (inside cells). Together with sodium (Na$^+$/K$^+$ pump), it maintains the electrochemical gradient across cell membranes needed for nerve impulse transmission, and it also helps activate several enzymes and regulate the regular beating of the heart.

Magnesium (Mg$^{2+}$)

Magnesium is present in bones and teeth and is a vital intracellular cation. It acts as a cofactor for a large number of enzymes, particularly those involved in energy (ATP) transfer, and it is a central constituent of the chlorophyll molecule in green plants, making it essential for photosynthesis. It also plays a role in neuromuscular function.

Calcium (Ca$^{2+}$) …

Discuss each ion's biological role separately, distinguishing the Na$^+$/K$^+$ pair (fluid balance and nerve/muscle signalling via the Na$^+$/K$^+$ pump) from the Mg$^{2+}$/Ca$^{2+}$ pair (structural roles …

• Mixing up which ion is predominantly extracellular (Na$^+$, Ca$^{2+}$) versus intracellular (K$^+$, Mg$^{2+}$).
• Omitting magnesium's crucial role in chlorophyll/photosynthesis, a distinctive and often-tested fact. …