Overview
This lecture covers the essential physical and chemical properties of water, emphasizing their biological significance and impact on living organisms.
Structure and Bonding in Water
- Water (Hâ‚‚O) has one oxygen and two hydrogen atoms bonded covalently.
- Electrons are shared unequally: oxygen is partially negative, hydrogens are partially positive, making water a polar molecule.
- Hydrogen bonds form between water molecules’ partial charges, depicted with dashed lines.
Cohesion and Adhesion
- Cohesion: attraction between water molecules due to hydrogen bonds; enables processes like transpiration in plants.
- Transpiration relies on water molecules pulling each other up through plant xylem.
- Surface tension, a result of cohesion, allows some organisms to live on the water’s surface.
- Adhesion: attraction between water molecules and solid surfaces, supporting capillary action in plant xylem.
Water as a Solvent
- Water dissolves hydrophilic (water-loving) substances: polar molecules (e.g., glucose) and ions.
- Hydrophobic (water-hating) substances like lipids and gases are not soluble in water.
- Metabolic reactions require solutes to be in solution, making water critical for life.
- Water’s solvent properties facilitate transport in plant xylem/phloem and animal blood plasma.
Buoyancy
- Water exerts an upward buoyant force, allowing less dense objects and organisms to float.
- Many aquatic organisms adjust buoyancy (e.g., fish use swim bladders) to control their position in water.
Viscosity
- Viscosity is water’s resistance to flow; it increases with solute concentration.
- Some viscosity is necessary for aquatic movement, but excessive viscosity can hinder flow in transport systems.
Thermal Properties
- Water is a good conductor of heat, enabling efficient heat transfer within and between organisms.
- Water’s high specific heat capacity means it resists rapid temperature changes, moderating environmental and body temperatures.
Biological Adaptations
- Aquatic and terrestrial organisms have adaptations (e.g., blubber, large ears) to manage water’s buoyancy, viscosity, and thermal conductivity.
Key Terms & Definitions
- Covalent bond — chemical bond sharing electron pairs between atoms.
- Polar molecule — a molecule with uneven electron distribution, creating partial charges.
- Hydrogen bond — a weak bond between oppositely charged parts of molecules.
- Cohesion — attraction between like molecules (water-water).
- Adhesion — attraction between different substances (water-solid).
- Solute — substance dissolved in a solvent.
- Solvent — liquid in which a solute dissolves.
- Solution — homogeneous mixture of solute and solvent.
- Hydrophilic — water-loving; soluble in water.
- Hydrophobic — water-hating; not soluble in water.
- Buoyancy — upward force exerted by fluid, allowing objects to float.
- Viscosity — resistance of a fluid to flow.
- Thermal conductivity — ability of a substance to conduct heat.
- Specific heat capacity — energy needed to raise temperature of a substance.
Action Items / Next Steps
- Review plant xylem and phloem structure and function.
- Study adaptations of aquatic and terrestrial organisms to water’s physical properties.
- Prepare for discussion on transpiration and water movement in plants.