AP Biology: Structure of Water and Hydrogen Bonding

Instructor Introduction

Understand the chemical structure of water
Recognize how water's structure results in polarity
Explain how water's polarity leads to cohesion and adhesion via hydrogen bonds
Identify chemical characteristics of water responsible for its cohesive and adhesive properties
Explore how living systems depend on water's chemical properties

Water is composed of:
- Elements: Oxygen and Hydrogen
- Ratio: 1:2 (H:O)
Bonds: Covalent bonds where electrons are shared
Electronegativity:
- Oxygen is more electronegative than hydrogen
- Leads to unequal electron sharing

Polarity: A molecule with dual charges
- Oxygen end is more negative
- Hydrogen end is more positive
Polar Covalent Bond: Due to polarity

Definition: Weak interactions between negative and positive regions of different molecules
Cohesion: Hydrogen bonds between same molecules (e.g., water-water)
Adhesion: Hydrogen bonds between different molecules (e.g., water-amino acid)

Surface Tension:
- Result of increased hydrogen bonds at the surface
- Example: Water droplet on penny, leaves floating on water
Solvency Abilities:
- Water as a solvent (not universal)
- Dissolves materials, important for nutrient access
Ice Floats:
- Solid water (ice) is less dense than liquid
- Aquatic life survives under ice in freezing climates
High Heat Capacity:
- Absorbs thermal energy without changing state
- Important for temperature regulation in aquatic organisms
Capillary Action:
- Due to cohesive and adhesive properties
- Allows plants to access water from soil