Overview
This lecture explains the critical role of water in biological systems, focusing on its molecular structure, chemical properties, and the importance of these properties to life and organisms.
Structure and Polarity of Water
- Water (Hâ‚‚O) consists of one oxygen atom covalently bonded to two hydrogen atoms.
- Water molecules are polar due to unequal electron sharing and an asymmetrical shape.
- The oxygen end is partially negative; the hydrogen ends are partially positive.
- Water’s polarity enables the formation of hydrogen bonds between molecules.
Cohesion and Adhesion
- Cohesion: Water molecules stick together due to hydrogen bonds, important for water movement in plants (xylem).
- Cohesion helps plants transport water from roots to leaves against gravity via transpiration.
- Surface tension results from cohesion, allowing insects like water striders to walk on water.
- Adhesion: Water molecules stick to other polar surfaces, aiding water movement through soil and plant tissues (capillary action).
Solvent Properties and Biological Importance
- Water is a universal solvent due to its polarity, dissolving ionic and polar substances (hydrophilic).
- Water dissolves salt (sodium chloride) by surrounding and separating ions.
- Hydrophobic (nonpolar) substances do not dissolve in water and contribute to structures like cell membranes.
- Water’s solvent ability enables metabolic reactions and transport of nutrients in blood and plant xylem.
Physical Properties of Water
- High specific heat: Water requires more energy to increase temperature, aiding temperature stability in organisms.
- High thermal conductivity: Water efficiently transfers heat, exemplified by blood cooling in animals.
- Buoyancy: Water’s density supports the floating of organisms like algae; gas vacuoles help algae adjust depth.
- Low viscosity: Water flows easily, but viscosity increases with dissolved solutes, influencing blood and ocean water flow.
Comparative Adaptations: Black-throated Loon vs. Ringed Seal
- Both species interact with water; the seal spends more time submerged, experiencing greater effects of water’s viscosity and heat loss.
- The seal has adaptations (thick fur) to retain heat and manage energy costs, while the loon faces less resistance and heat loss in air.
Key Terms & Definitions
- Polar molecule — a molecule with uneven charge distribution, like water.
- Hydrogen bond — a weak bond between a hydrogen atom and an electronegative atom.
- Cohesion — attraction between similar molecules, e.g., water to water.
- Adhesion — attraction between different substances, e.g., water to xylem walls.
- Surface tension — the tightness on the water surface due to cohesion.
- Capillary action — movement of liquid within narrow spaces due to adhesion and cohesion.
- Solvent — substance that dissolves others; in biology, water is the primary solvent.
- Hydrophilic — attracted to water; dissolves in water.
- Hydrophobic — repelled by water; does not dissolve in water.
- Specific heat — heat required to raise the temperature of a substance by 1°C.
- Thermal conductivity — ability of a material to conduct heat.
- Buoyancy — upward force allowing objects to float.
- Viscosity — resistance to flow.
Action Items / Next Steps
- Review examples comparing the ringed seal and black-throated loon for IB exam prep.
- Study how water’s properties affect other organisms mentioned in the curriculum.