Overview
This lecture covers the properties of water, focusing on its molecular structure, bonding, and significance for biological and environmental processes, especially in living organisms.
Structure and Bonding in Water
- Water is a liquid at most Earth temperatures, enabling metabolic reactions in cells.
- Each water molecule (H₂O) consists of one oxygen and two hydrogen atoms bonded covalently.
- The oxygen atom pulls electrons closer, creating partial charges (oxygen: partial negative, hydrogen: partial positive).
- Hydrogen bonds form between the partially positive hydrogen of one molecule and the partially negative oxygen of another.
Cohesion and Adhesion
- Cohesion is the attraction between water molecules due to hydrogen bonds, causing water to stick together.
- Cohesion aids in transpiration, allowing water to be pulled up through plant xylem.
- Surface tension, from cohesion, enables insects to walk on water without breaking the surface.
- Adhesion is the attraction between water molecules and solid surfaces, important for capillary action in plant vascular tissue.
Water as a Solvent
- Water dissolves many substances (solvent), forming solutions with solutes (substances dissolved).
- Water dissolves hydrophilic (water-loving) substances, which are polar molecules or ions.
- Hydrophobic (water-hating) substances (e.g. lipids) are not soluble in water.
- Water’s solvent properties make it a vital transport medium in plants (xylem and phloem) and animals (blood plasma).
Physical Properties: Buoyancy and Viscosity
- Water provides buoyancy, exerting an upward force that helps less dense objects and aquatic organisms float.
- Some organisms, like fish, adjust buoyancy using organs such as swim bladders.
- Viscosity is water’s resistance to flow; it increases with more dissolved substances, affecting transport and movement in water.
Thermal Conductivity and Specific Heat Capacity
- Water conducts heat well, enabling efficient heat transfer in aquatic environments and organisms.
- Water’s high specific heat capacity means it absorbs a lot of energy before changing temperature, moderating temperature fluctuations in aquatic habitats and organisms.
Adaptations to Water’s Properties
- Aquatic organisms exhibit adaptations (e.g. blubber in seals) to deal with water’s thermal conductivity and viscosity.
- Terrestrial and aquatic organisms adapt differently to the contrasting properties of water and air.
Key Terms & Definitions
- Covalent bond — Sharing of electron pairs between atoms.
- Hydrogen bond — Attraction between a hydrogen atom with partial positive charge and an electronegative atom with partial negative charge in different molecules.
- Cohesion — Attraction between molecules of the same substance (water to water).
- Adhesion — Attraction between molecules of different substances (water to solid surface).
- Capillary action — Movement of water within narrow spaces due to adhesion and cohesion.
- Solute — Substance dissolved in a solvent.
- Solvent — Liquid in which a solute is dissolved to form a solution.
- Hydrophilic — Attracted to water; usually polar or charged.
- Hydrophobic — Repelled by water; usually nonpolar.
- Buoyancy — Upward force that allows objects to float in fluid.
- Viscosity — Resistance of a fluid to flow.
- Thermal conductivity — Ability of a substance to conduct heat.
- Specific heat capacity — Amount of heat energy required to change the temperature of a substance.
Action Items / Next Steps
- Review plant transpiration and xylem structure in detail.
- Practice drawing and labeling water molecules, including covalent and hydrogen bonds.
- Learn examples of hydrophilic and hydrophobic molecules relevant to biology.