| 1.1 Structure of Water and Hydrogen Bonding |
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Summary
Water's biological importance stems from its molecular polarity. Because oxygen is more electronegative than hydrogen, the shared electrons in each polar covalent O–H bond are pulled toward oxygen, giving oxygen a partial negative charge (δ–) and each hydrogen a partial positive charge (δ+). These partial charges enable hydrogen bonding between water molecules and between water and other polar substances.
Thermal Properties
- These hydrogen bonds give water a high specific heat capacity, which buffers organisms against rapid temperature changes and supports homeostasis.
- Water's high heat of vaporization allows evaporative cooling, a mechanism organisms use to regulate body temperature.
Both thermal properties depend directly on the energy required to disrupt hydrogen bonds.
Hydrogen bonding among water molecules also produces cohesion, adhesion, and surface tension — three properties with direct biological consequences.
Cohesion
- Cohesion holds water molecules together, maintaining continuous water columns in plant xylem.
Adhesion
- Adhesion allows water to cling to polar surfaces such as vessel walls, assisting upward transport (capillary action).
Surface Tension
- Surface tension, caused by unbalanced hydrogen bonding at the air-water interface, creates a resistant surface film that supports small organisms.
Every major water property tested on the AP exam traces back to polarity and hydrogen bonding, so building explanations from that molecular foundation is essential.