Preview
IB DP Biology — A: Unity and Diversity | SL | A1.1
📋 What You Will Learn
- Identify the structural features of a water molecule that give rise to its unique properties — polarity, hydrogen bonding, and the arrangement of partial charges
- Explain how cohesion, adhesion, thermal properties, and the solvent capacity of water arise directly from hydrogen bonding
- Analyse data on the physical properties of water and relate anomalous values (e.g. high boiling point, maximum density at 4 °C) to biological significance
- Apply understanding of water's properties to explain biological phenomena — from transpiration in plants to temperature regulation in organisms
- Evaluate why water's unique combination of properties makes it the universal medium for life as we know it
🔍 Guiding Question How does the structure of the water molecule give rise to the properties that make water essential to life?
🌍 Why This Matters A water strider walks on the surface of a pond without breaking through. A cactus survives weeks without rain because water clings to soil particles. A fever of just 2 °C above normal can impair enzyme function. Every one of these phenomena traces back to a single molecule — and a single bond type that should not, by any chemical logic, be as powerful as it is.
The Structure of Water: Where Everything Begins
Before water can do anything biologically remarkable, it has to be structurally unusual. Most small molecules with a molar mass around 18 g mol⁻¹ are gases at room temperature. Water is a liquid. That anomaly is the first clue that something extraordinary is happening at the molecular level.
A water molecule consists of one oxygen atom covalently bonded to two hydrogen atoms. The oxygen atom is far more electronegative than hydrogen — it pulls the shared electrons toward itself with much greater force. This unequal sharing of electrons creates regions of partial charge across the molecule.
📖 Key Definition — Polar Molecule A polar molecule is one in which the distribution of electrons is unequal, creating regions of partial positive charge (δ+) and partial negative charge (δ−). In water, the oxygen end carries a δ− charge and each hydrogen carries a δ+ charge.
The two O–H bonds are not arranged in a straight line. The molecule is bent — the bond angle is approximately 104.5°. This geometry, combined with the two lone pairs of electrons on oxygen, means the partial charges do not cancel out. The molecule has a permanent dipole. This is the foundation of everything that follows.
📖 Key Definition — Hydrogen Bond A hydrogen bond is an electrostatic attraction between a δ+ hydrogen atom on one molecule and a δ− electronegative atom (oxygen, nitrogen, or fluorine) on a neighbouring molecule. In water, each molecule can form up to four hydrogen bonds — two through its hydrogen atoms and two through its lone pairs on oxygen.
Hydrogen bonds are not covalent bonds. They are weaker — roughly 20 times weaker than a covalent O–H bond. But they are persistent, numerous, and constantly reforming. In liquid water at any given moment, billions of hydrogen bonds exist simultaneously across the bulk of the liquid. Their collective effect is