AP Biology - Topic 1.1: Structure of Water and Hydrogen Bonding

Introduction

• Unit: Chemistry of Life
• Water is essential for all life forms; most organisms are largely composed of water.
• Unique properties of water make it essential; it behaves differently compared to other molecules.

Water Molecule Structure

• Polarity: Water is a polar molecule with two sides (positive and negative).
  • Water molecule = H2O (2 Hydrogen, 1 Oxygen).
  • Arranged in a "Mickey Mouse" shape.
  • Polarity due to different electronegativities: Hydrogen (low), Oxygen (high).
  • Results in slight positive and negative charges on opposite sides of the molecule.

Hydrogen Bonds

• Definition: Attractions between water molecules due to polarity.
• Not actual bonds but interactions between molecules.
• Constantly forming and breaking due to water's fluidity.
• Multiple hydrogen bonds lead to strong collective interactions.

Unique Properties of Water

1. Cohesion

   • Water molecules stick to each other due to hydrogen bonding.
   • Visible in the formation of water droplets.

2. Adhesion

   • Water molecules stick to other polar surfaces.
   • Critical for plant function: water moves up through xylem by adhering to cell walls.

3. Surface Tension

   • Measure of how difficult it is to break the surface of a liquid.
   • Allows insects like water striders to glide on water surfaces.
   • High surface tension due to hydrogen bonds.

Conclusion

• Cohesion, adhesion, and surface tension are explained by water's ability to form hydrogen bonds due to its polar nature.
• The atomic structure significantly influences the properties of water at a larger scale.

Next Steps

• Stay tuned for Topic 1.2 in the next lecture.

• For questions, reach out to Mr. Poser.