Water: Molecular Structure of Water (With Diagram)

Water is the most abundant inorganic compound in living systems. The water content of an organism is related to its age and metabolic activity.

For example, water amount is highest in embryo (90-95%) and decrease progressively in adult and in the aged.

Water constitutes 70-90% of living cells. Nearly 90-95% of succulent plant body is water. In human, about two-third of the body composed of water and of these, about 55 %(20-22 liters) is confined within cells as intracellular fluid (ICF) and the remaining 45% exist as extracellular fluid (ECF) like blood, lymph, tissue fluid etc.

In the cell water exist in two forms i.e. free and bound. Free water constitutes 95% of the total cellular water which is used as a natural solvent. Bound water represents just 4-5% of total cellular water which is loosely held to biomolecules by hydrogen bonds and other forces.

Molecular Structure of Water:

The Water (H₂O) molecule has a triangular geometry with O-H bond distance of 0.0965nm and the H-O-H bond angle is 104.5°. Although the water as a whole is electrically neutral, it behaves as an electrical dipole. This is because; oxygen atom is more electronegative than the hydrogen atoms, so it attracts electrons away from the hydrogen nuclei. This gives the oxygen atom two partial negative charges (δ-), while each hydrogen atom carries a partial positive charge (δ+).

In pure liquid water, each H₂O molecule is hydrogen bonded to an average of 3.4 adjacent H₂O molecules, compared with 4 in ice. Hydrogen bond is a week non-covalent bond, in which a hydrogen atom is shared by two other atoms i.e. hydrogen donor (tightly bound to hydrogen) and hydrogen acceptor. In biological systems, the hydrogen donor is an oxygen atom (as in water) or nitrogen atom (as in protein or in nitrogen base pairs).

Molecules of water join together transiently in a hydrogen-bonded lattice. Even at 37°C, C, 15% of the water molecules is joined to four others in a short-lived assembly known as a “flickering cluster.” The cohesive nature of water is responsible for many of its unusual properties, such as high surface tension, specific heat, and heat of vaporization.

The length of hydrogen bond is about 0.26 to 0.31 nm which is intermediate between a covalent bond and a Vander Waals bond. The energy of hydrogen bond in water is about 5 Kcal/mol, which is stronger than Vander Waal’s bond (1 Kcal/mol) and weaker than a covalent H-O bond (1 10 Kcal/mol).

When inorganic salts are dissolved in water, they dissociated into cations and anions. Each ion is surrounded by a layer of oriented water dipoles called a hydration shell. The thickness of hydrogen shell is greater for smaller ions than for larger ones carrying the same quantity of electrical charge.