Overview
This lecture explains the unique chemical and physical properties of water and why these features are vital for biological systems.
Structure and Polarity of Water
- Water (Hâ‚‚O) is a polar molecule shaped like an outstretched V.
- Oxygen is highly electronegative, attracting electrons closer and creating a partial negative charge on oxygen and a partial positive charge on hydrogens.
- Water molecules form hydrogen bonds between the slightly positive hydrogen of one molecule and the slightly negative oxygen of another.
Cohesion, Adhesion, and Surface Tension
- Cohesion is the attraction between water molecules via hydrogen bonds, causing phenomena like surface tension.
- Adhesion is water’s ability to stick to other surfaces, such as plant xylem walls, aiding upward movement.
- Cohesion and adhesion allow water to move against gravity in plants and support surface-tension-walking animals like water striders.
Solvent Properties
- Water’s polarity makes it an excellent solvent, able to dissolve many polar molecules and ionic compounds.
- This property is essential for many biological processes, including kidney function and bodily fluids.
Density and Ice Formation
- Unlike most substances, water expands and becomes less dense when it freezes due to hydrogen bonding.
- Ice forms a floating, insulating layer that protects aquatic life during freezing temperatures.
Temperature Regulation
- Water has a high specific heat, meaning it resists rapid temperature changes and stabilizes environments.
- This property allows water to absorb or release heat, buffering temperature variations for organisms.
Evaporative Cooling
- Evaporation removes heat as higher-energy water molecules leave the surface, cooling organisms.
- Both animals (e.g., sweating) and plants use evaporative cooling to regulate temperature and protect cellular processes.
Key Terms & Definitions
- Polar molecule — a molecule with uneven distribution of charge, resulting in partial positive and negative ends.
- Electronegativity — the ability of an atom to attract shared electrons.
- Hydrogen bond — a weak bond between a hydrogen atom and a more electronegative atom (like oxygen).
- Cohesion — attraction between molecules of the same substance (water to water).
- Adhesion — attraction between molecules of different substances (water to plant walls).
- Surface tension — the cohesive force at the surface of a liquid.
- Solvent — a substance that dissolves other substances.
- Specific heat — the heat required to change the temperature of 1g of a substance by 1°C.
Action Items / Next Steps
- Review examples of water’s unique properties in your textbook.
- Watch the plant xylem transport video for more details on water movement in plants.