Overview
This lecture covers the polarity of water, hydrogen bonding, their roles in biological molecules, and explains water's unique properties, as well as the concepts of acids, bases, and pH.
Water Polarity and Hydrogen Bonding
- Water is a polar molecule due to unequal electron sharing between oxygen (partial negative) and hydrogen (partial positive).
- Hydrogen bonds are weak intermolecular bonds that form between the partially positive hydrogen of one water molecule and the partially negative oxygen of another.
- Hydrogen bonds are much weaker than covalent or ionic (intramolecular) bonds.
Biological Importance of Hydrogen Bonds
- Hydrogen bonds are not unique to water; they also occur in DNA (between nitrogenous bases) and in proteins and RNA.
- In DNA, hydrogen bonds hold together nitrogenous bases like adenine and thymine.
- Hydrogen bonds are essential for maintaining the structure of many biological molecules.
Properties of Water Resulting from Hydrogen Bonding
- Cohesion is the attraction between water molecules via hydrogen bonding, leading to high heat of vaporization and high specific heat.
- Adhesion is water's ability to stick to other substances (e.g., plant cell walls), enabling capillary action and transpiration in plants.
- Surface tension is the result of cohesive forces at the water's surface, allowing objects to rest on it without sinking.
Acids, Bases, and pH
- Acidic solutions have more hydrogen ions (H⁺) than hydroxide ions (OH⁻), resulting in a pH below 7.
- Basic solutions have more hydroxide ions than hydrogen ions, resulting in a pH above 7.
- Understanding pH is essential for AP Biology, though direct pH questions are rare.
Key Terms & Definitions
- Polar Molecule — A molecule with uneven distribution of charge, resulting in partial positive and negative ends.
- Hydrogen Bond — A weak bond between the partially positive hydrogen of one molecule and the partially negative atom of another.
- Cohesion — Attraction between molecules of the same substance (e.g., water molecules).
- Adhesion — Attraction between molecules of different substances.
- Surface Tension — The elastic “skin” on the surface of water resulting from cohesive forces.
- Acidic Solution — A solution with more hydrogen ions than hydroxide ions (pH < 7).
- Basic Solution — A solution with more hydroxide ions than hydrogen ions (pH > 7).
- pH — A numerical scale used to measure how acidic or basic a solution is.
Action Items / Next Steps
- Review hydrogen bonding in biological molecules like DNA and proteins.
- Practice identifying acids, bases, and neutral solutions using pH values.
- Watch the next video or complete interactive tutorials on water and hydrogen bonding.