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Chapter 3 Notes: Water and Life

CONCEPT 3.1: Polar covalent bonds in water molecules result in hydrogen bonding

  • Water (H₂O) is a polar molecule because electrons in the polar covalent bonds spend more time near oxygen than hydrogen.
    • Result: Oxygen has a partial negative charge (δ−); hydrogen atoms have partial positive charges (δ+).
    • Overall, the molecule is polar with uneven charge distribution.
  • Polarity enables water molecules to form hydrogen bonds with each other (weak attractions between opposite charges).
  • Consequences of hydrogen bonding:
    • Water molecules can bond to form a cohesive network essential for many properties.
    • Hydrogen bonds are the basis for water’s emergent properties discussed in Concept 3.2.
  • Significance:
    • Hydrogen bonding underpins water’s solvent abilities, surface tension, and thermal properties that support life.

CONCEPT 3.2: Four emergent properties of water contribute to Earth’s suitability for life

  • Four emergent properties (from water’s structure and hydrogen bonding):
    • Cohesion
    • Ability to moderate temperature
    • Expansion upon freezing
    • Versatility as a solvent
  • Cohesion of Water Molecules
    • Cohesion: water molecules are held together by hydrogen bonds.
    • Results in high surface tension (difficulty of stretching/breaking the surface).
    • Role in plants: cohesion helps transport water and dissolved nutrients against gravity.
  • Adhesion (attraction between different substances)
    • Example: water’s attraction to plant cell walls.
    • Helps counter downward pull of gravity, aiding movement of water in plant tissues.
  • Moderation of Temperature by Water
    • Water absorbs heat from warmer air and releases stored heat to cooler air.
    • A large amount of heat can be absorbed/released with only a small change in water’s temperature.
  • Temperature and Heat (definitions and relationships)
    • Kinetic energy: energy of motion.
    • Thermal energy: kinetic energy of random motion of atoms/molecules.
    • Temperature: average kinetic energy of molecules in a body of matter.
    • Heat: thermal energy transferred from one body to another.
    • Calorie (cal): amount of heat required to raise the temperature of 1 g of water by 1°C; also the amount of heat released when 1 g of water cools by 1°C.
    • Food energy units: kilocalorie (kcal) = 1,000 cal; on nutrition labels, "Calories" are kilocalories.
    • Key conversions:
    • 1\; \text{cal} = 4.184\; \text{J}
    • 1\; \text{kcal} = 1000\; \text{cal}
    • 1\; \text{J} = 0.239\; \text{cal}
  • Water’s High Specific Heat
    • Specific heat of water: c = 1\;\text{cal}\,/(\text{g} \cdot ^\circ\text{C})
    • Water resists temperature changes due to hydrogen bonding: heat is absorbed when bonds break; heat is released when bonds form.
    • This high specific heat minimizes temperature fluctuations, enabling life to persist in a relatively stable environment.
  • Implications for climate and organisms
    • Large bodies of water can absorb/store heat from the sun, warming little and stabilizing coastal climates.
    • Evaporation and surface cooling help regulate temperatures in organisms and bodies of water.
    • Example: coastal areas enjoy moderated temperatures due to the ocean.

CONCEPT 3.3: Acidic and basic conditions affect living organisms

  • A hydrogen atom in a hydrogen bond between two water molecules can shift from one molecule to the other.
    • The hydrogen (proton) is transferred as H⁺; the molecule that loses the proton becomes OH⁻ (hydroxide).
    • The molecule that gains the proton becomes H₃O⁺ (hydronium); often represented as H⁺ in simplified form.
  • Acids and Bases
    • An acid increases H⁺ concentration in a solution.
    • A base reduces H⁺ concentration in a solution.
    • Strong acids/bases dissociate completely in water.
    • Weak acids/bases reversibly release/accept hydrogen ions but can shift the H⁺/OH⁻ balance away from neutrality.

THE pH SCALE

  • At 25°C, the product of H⁺ and OH⁻ concentrations is constant: [\mathrm{H}^+][\mathrm{OH}^-] = 10^{-14}.
  • The pH of a solution is defined as: \text{pH} = -\log [\mathrm{H}^+].
  • Neutral water: [\mathrm{H}^+] = 10^{-7}, so \text{pH} = 7.
  • Acidic solutions have \text{pH} < 7; basic (alkaline) solutions have \text{pH} > 7.
  • Most biological fluids: pH values typically in the range 6 \text{ to } 8.

BUFFERS

  • Internal pH of most living cells is close to 7.
  • Buffers minimize changes in concentrations of H⁺ and OH⁻ in a solution.
  • Most buffer solutions contain a weak acid and its conjugate base, which can reversible bind to H⁺ ions.
  • Buffers help maintain homeostasis of pH in biological systems.

ADDITIONAL CONTEXT AND REAL-WORLD CONNECTIONS

  • Real-world relevance of water’s properties:
    • Cohesion/adhesion underpin transport of water in plants (e.g., from roots to leaves).
    • Water’s high heat capacity stabilizes climate and protects organisms from rapid temperature changes.
    • Ice floats due to lower density than liquid water, preventing entire bodies of water from freezing solid and enabling life to persist under ice.
    • Water as a universal solvent enables dissolution and chemical reactions essential to life; hydration shells facilitate ion transport.
    • pH balance and buffering systems are critical for cellular processes, enzyme activity, and metabolic pathways.
  • Foundational principles linking to earlier chapters/ideas:
    • Molecular polarity and hydrogen bonding explain macroscopic properties like cohesion, adhesion, surface tension, and solvent power.
    • Energy concepts (kinetic energy, thermal energy, heat) connect molecular interactions to temperature changes and energy transfer in systems.
    • Molarity and Avogadro’s number underpin quantitative chemistry used in biology experiments and metabolic calculations.

KEY FORMULAS AND CONCEPTS TO REMEMBER

  • Hydrogen bonding and polarity concepts:
    • Polar covalent bonds lead to molecular polarity and hydrogen bonding between water molecules.
  • Water’s emergent properties:
    • Cohesion and adhesion contribute to surface tension and plant transport.
    • High specific heat and high heat of vaporization explain temperature regulation and evaporative cooling.
    • Ice expands upon freezing, leading to ice floating and insulating effects.
  • Solvent and solution terminology:
    • Solution: homogeneous mixture of substances.
    • Solvent: dissolving agent.
    • Solute: substance dissolved.
    • Aqueous solution: solvent is water.
    • Hydration shell: water molecules surrounding dissolved ions.
  • Hydrophilic vs hydrophobic:
    • Hydrophilic substances have an affinity for water; hydrophobic substances resist water.
    • Oils are hydrophobic; major components of cell membranes.
  • Solute concentration and quantities:
    • Molecular mass is the sum of atomic masses in a molecule.
    • 1 mole = 6.02 \times 10^{23} molecules.
    • Avogadro’s number and the dalton definition: 6.02 \times 10^{23} \text{ daltons} = 1 \text{ g}.
    • Molarity: M = \frac{\text{moles solute}}{\text{liter of solution}}.
  • Acid-base chemistry and pH:
    • Hydrogen ion transfer in water forms H⁺/OH⁻ or H₃O⁺ in solution.
    • pH calculations: \text{pH} = -\log [\mathrm{H}^+] and [\mathrm{H}^+][\mathrm{OH}^-] = 10^{-14} at 25°C.
  • Energy units and conversions:
    • 1\; \text{cal} = 4.184\; \text{J}
    • 1\; \text{kcal} = 1000\; \text{cal}
    • 1\; \text{J} = 0.239\; \text{cal}
  • Temperature-related terms:
    • Specific heat: the amount of heat required to raise 1 g of a substance by 1°C.
    • Water’s specific heat: c = 1\;\text{cal}/(\text{g} \cdot { }^{\circ}\text{C}).
    • Heat vs. temperature: heat is energy transfer; temperature is average kinetic energy.