Structure of water and hydrogen bonding

Summary

Water's biological importance stems from its molecular polarity. The polar covalent bonds between oxygen and hydrogen create partial charges that 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.

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.

Surface Tension

• Surface tension, caused by unbalanced hydrogen bonding at the air-water interface, creates a resistant surface film that supports small organisms.

Hydrogen bonding among water molecules also produces cohesion, adhesion, and surface tension — three properties with direct biological consequences.

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.