What is The water molecule? — Biology

Definition 8.1 University Biology — Year 1 · Chapter 8 — Water and Small Biomolecules

Water, $\mathrm{H_2O}$ , is a bent molecule (the H–O–H angle is $104.5^\circ$ ) in which the oxygen draws the shared electrons toward itself: it is polar, with a partial negative charge on the oxygen and partial positive charges on the hydrogens. Each molecule can form up to four hydrogen bonds — an electrostatic attraction between a hydrogen bound to an electronegative atom (O, N) and a lone pair of another electronegative atom — two through its hydrogens, two through its oxygen. Liquid water is a network of such bonds, each lasting a few picoseconds.

A water molecule (H–O–H angle 104.5) and its four hydrogen-bonded neighbours. The polar bent molecule donates two hydrogen bonds through its hydrogens and accepts two on its oxygen; the network they form gives water its cohesion, its heat capacity and its power as a solvent. — A water molecule (H–O–H angle $104.5^\circ$ ) and its four hydrogen-bonded neighbours. The polar bent molecule donates two hydrogen bonds through its hydrogens and accepts two on its oxygen; the network they form gives water its cohesion, its heat capacity and its power as a solvent.

A water strider on a pond: the dimples under its feet are the surface held taut by the cohesion of hydrogen-bonded water.

Examples

Example 8.3 (The hydrophobic effect at work)

Shake oil into water and within minutes the droplets have merged: water squeezes out what it cannot bond to. The same effect folds a protein (its oily residues hide inside, Chapter 12), assembles a membrane (the tails of the lipids gather away from water, Chapter 9), and drives two matching molecular surfaces together. Nothing attracts the oil to the oil; water pushes.

Example 8.6 (Melting a double helix)

The two strands of DNA are held by two or three hydrogen bonds per base pair plus the stacking (van der Waals) of neighbouring pairs; a single pair would separate at once, but a thousand pairs in a row hold until the temperature reaches about $90\,{}^{\circ}\mathrm{C}$ , and reunite, in exactly the same register, when it is lowered (Chapter 11). Specific, strong in number, reversible.

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