Structure of Water and Hydrogen Bonding - AP Bio Study Guide 2024

Water

• Polar Molecule: Water is polar with an unequal distribution of charge; comprises two hydrogen atoms (partial positive charge) and one oxygen atom (partial negative charge).
• Polarity: Polar substances attract each other; nonpolar substances do not mix with polar substances (e.g., oil and water).
• Interactions:
  • Hydrophilic: Substances attracted to water.
  • Hydrophobic: Substances that avoid water.
  • Example: Lipids are hydrophobic due to nonpolar bonds.

Hydrogen Bonding

• Definition: An intermolecular bond where hydrogen bonds with electronegative atoms (oxygen, nitrogen, fluorine).
• Role in Biology:
  • Critical in molecules like water, DNA, proteins.
  • Responsible for molecular properties like protein shape, stability of compounds, and intermolecular interactions.
  • Gives water cohesion, adhesion, surface tension, specific heat, and evaporative cooling properties.

Properties of Water

• Cohesion: Attraction between water molecules due to hydrogen bonding. Supports water and nutrient transport in plants.
• Adhesion: Water’s attraction to other substances (e.g., water on a glass surface).
• Surface Tension: Difficulty breaking the surface of water due to cohesive forces (e.g., water striders).
• Specific Heat: High specific heat allows water to absorb or release large amounts of heat with little temperature change.
• Evaporative Cooling: High heat of vaporization absorbs heat, cooling surfaces (e.g., perspiration).
• Dissociation of Water:
  • Acid increases hydronium concentration.
  • Base increases hydroxide concentration.
  • pH scale: 0-14, with 7 as neutral; most biological fluids are 6-8.

Importance of Water

• Essential for all living organisms, supporting life through its unique properties.
• Polar and hydrogen bonding abilities enable water to support life at cellular and ecosystem levels.

Key Terms

• Acid: Donates hydrogen ions, increasing hydronium concentration.
• Adhesion: Attraction between different molecules.
• Base: Accepts protons or electrons; pH > 7.
• Chemical Formula: Represents atoms in a compound.
• Cohesion: Tendency of molecules of the same substance to stick together.
• Covalent Bonds: Sharing of electron pairs between atoms.
• Dipole Moment: Indicates polarity of a molecule.
• Dissociation of Water: Water breaking into ions (H+ and OH-).
• Electronegative Atom: Attracts electrons in a bond.
• Evaporative Cooling: Cooling by evaporation.
• Hydrogen: First element, forms water.
• Hydrogen Ions: Crucial in acid-base reactions.
• Hydrogen Bonding: Attraction involving hydrogen.
• Hydronium Concentration: Measure of hydronium ions, determines pH.
• Hydrophobic Substance: Does not mix with water.
• Hydrophilic Substance: Mixes with water.
• Logarithmic: Scale increasing by factors of 10.
• Neutral pH: Value of 7, neither acidic nor basic.
• Oxygen: Essential element in air and water.
• Partial Charges: Results from unequal electron sharing.
• pH Scale: Measures acidity/alkalinity.
• Polar Molecule: Uneven charge distribution.
• Specific Heat: Energy required to change temperature.
• Surface Tension: Cohesive force at the liquid surface.
• Transpiration: Water vapor escape from plants.
• Water Molecules: Composed of two hydrogen and one oxygen atom.