Water in Biology – Comprehensive Study Notes (Hydrolysis, Water Compartments, and Properties)

Study Strategy and Exam Prep

  • Practice exams are tools to assess what you know and what you don’t know.
  • Study the material first, then take practice exams.
  • After grading an exam, review which questions you got right and wrong to identify topics to revisit.
  • Revisit the chapter material, ask questions, and use office hours as needed.

Focus: Water in Biology

  • Humans are made mostly of water; water compartments exist inside and outside cells.
  • Cells rely on metabolism, i.e., all the chemical reactions occurring within them.
  • Reactions begin with reactants, bonds in reactants are broken, atoms rearrange to form products.
  • Law of conservation of matter: atoms are conserved; the same number of atoms on the left and right, though forms may differ.
  • Water can be a reactant or a product in cellular reactions.

Key Chemistry Terms (introduced)

  • Hydrolysis: a reaction in which water is used to break bonds in reactants.
    • General form: ext{Reactant} + ext{H}_2 ext{O}
      ightarrow ext{Products}
    • Bonds are broken, atoms rearrange into new products.
  • Dehydration synthesis: bonds are formed to create a larger molecule, and water is produced as a byproduct.
    • General form: ext{Reactants}
      ightarrow ext{Product} + ext{H}_2 ext{O}
  • Water as a reactant or product connects to life’s chemistry (metabolism) in cells.

Cellular Water Compartments

  • Extracellular fluid (ECF): the watery environment outside cells; also called interstitial fluid.
  • Cytosol: the fluid portion of the cytoplasm inside cells (the liquid part bathing organelles).
  • Cytoplasm vs cytosol:
    • Cytoplasm = everything inside the cell membrane (fluid plus organelles).
    • Cytosol = the liquid (water) portion of the cytoplasm; does not include organelles.
  • Water compartments matter because chemical reactions occur both inside cells (cytosol) and outside cells (ECF).
  • Osmosis: movement of water across a semipermeable membrane to balance solute concentrations (water movement is central to cell size and shape).
  • Plant cells: water movement affects turgor pressure; plants can be described as turgid or flaccid depending on water status.

Hydrogen Bonds and Water's Polarity

  • Water is a polar molecule:
    • Oxygen is more electronegative than hydrogen (approx.
      ext{O} ext{ electronegativity}
      ightarrow 3.5,
      ext{H}
      ightarrow 2.1
    • This creates partial charges: ext{O is partially negative} \ ext{H atoms are partially positive}
  • Water can form up to four hydrogen bonds with neighboring water molecules.
  • Hydrogen bonds are weaker than covalent bonds but are crucial for water’s properties and biology.
  • Intermolecular hydrogen bonds (between molecules) and intramolecular covalent bonds (within a molecule) co-exist in liquid water.
  • Water’s polarity leads to the principle: “like dissolves like.”
    • Polar solvents dissolve polar solutes; nonpolar solvents dissolve nonpolar solutes.

Solvent, Solute, and Dissociation

  • Solvent: the dissolving medium (water, in this context).
  • Solute: the substance being dissolved (e.g., NaCl, sugar).
  • Hydration shells: water molecules surround ions when Ionic compounds dissolve.
    • Example: Sodium chloride (NaCl) dissociates in water into Na⁺ and Cl⁻; water orients with O atoms around Na⁺ and H atoms around Cl⁻.
    • Salt dissolves because of water’s polarity; the process is called dissociation when ions separate.
  • Salts dissolve because water is polar; the salt is the solute; water is the solvent.
  • Glucose/sugar terminology:
    • In everyday language: sugar may refer to glucose or sucrose; technically, glucose is a simple sugar (monosaccharide) and sucrose is a disaccharide; in common lab talk, “sugar” often means glucose or sucrose depending on context.
  • Hydrophobic vs hydrophilic:
    • Hydrophilic: water-loving; polar substances dissolve in water.
    • Hydrophobic: water-fearing; nonpolar substances (like oils) do not dissolve in water.
  • Hydration and dissolution depend on the solute’s polarity and the solvent’s polarity.

Ionic Dissociation and Solubility in Water

  • When NaCl is added to water, it dissociates into Na⁺ and Cl⁻, surrounded by water molecules (hydration shells).
  • Water can dissolve many ionic compounds due to polarity; nonpolar substances like oil do not dissolve well in water (hydrophobic).
  • Solutions: a solvent with dissolved solute; examples include caffeine solutions and ethanol-water mixtures used in lab practice.

Water as a Regulator: Heat, Cohesion, and Adhesion

  • Water has a high heat capacity (specific heat) and a high heat of vaporization due to extensive hydrogen bonding.
    • Example from lecture: 100 calories of heat raise 100 g of water by 1°C.
  • Heat of vaporization and high heat capacity provide a temperature buffer for organisms and environments.
  • Evaporation of sweat uses water’s high heat of vaporization to cool the body.
  • Cohesion vs adhesion:
    • Cohesion: water molecules attract each other (water to water).
    • Adhesion: water molecules stick to surfaces (water to other materials like glass or plastics).
    • Both properties are driven by hydrogen bonding and are essential for processes like capillary action in plants and surface interactions in biology.
  • Surface tension: collective hydrogen bonding creates a strong surface layer; water striders float due to high surface tension.
  • Capillary action in plants relies on cohesion and adhesion to move water from roots to leaves.

States of Water and Phase Changes

  • Water exists in three states: solid (ice), liquid, and gas (vapor).
  • Phase changes require energy input or release:
    • Solid to liquid (melting) and liquid to gas (vaporization) require energy input (endothermic).
    • Gas to liquid (condensation) and liquid to solid (freezing) release energy (exothermic).
  • Heat concepts:
    • Specific heat (the energy required to raise the temperature of a substance by 1°C): water has a high specific heat.
    • Heat of vaporization (energy required to convert a liquid to a gas at its boiling point).
  • Practical example: When heating a cup of water on a burner, liquid water remains at 100°C while it boils; the added energy goes into breaking hydrogen bonds and forming steam rather than increasing temperature of the liquid.
  • Ice density vs liquid water:
    • Ice (solid water) is less dense than liquid water due to hexagonal hydrogen-bond lattice; this causes ice to float on liquid water.
    • This property has major ecological implications (e.g., insulating aquatic environments, climate effects).

Colligative Properties (Depend on Solute Quantity, Not Identity)

  • Colligative properties depend on the number of dissolved particles (solutes), not on their specific identity.
  • Increasing solute particles alters properties such as:
    • Boiling point (boiling point elevation) and freezing point (freezing point depression).
    • Vapor pressure and osmotic effects (osmotic pressure).
    • Antifreeze in car engines uses solutes to lower freezing point and control boiling behavior to prevent freezing.
  • Antifreeze example illustrates how solutes modify water’s properties by changing colligative properties.

Preliminary to pH: Hydrogen Ions and pH Scale

  • pH stands for potential hydrogen.
  • A key reaction is the breaking of a polar covalent bond between oxygen and hydrogen, where the hydrogen can be released as a hydrogen ion (H⁺) when the electron remains with the more electronegative atom (oxygen).
    • This leaves a hydrogen ion (a proton) behind, which is a hydrogen ion or a proton.
  • Brackets around a species denote its concentration, e.g., [H⁺].
  • pH scale ranges from 0 to 14, with 7 being neutral in aqueous solutions.
  • Higher [H⁺] means more acidic; lower [H⁺] means more basic (alkaline).
  • A formal, widely used expression (to be applied in pH discussions):
    • ext{pH} = - ext{log}_{10}[H^+]

Connections to Broader Topics

  • Chemistry basics underpin biology: structure–function relationships in cells depend on molecular interactions (e.g., hydrogen bonds influence protein folding, DNA base pairing, and transport across membranes).
  • The biological hierarchy: water and solutes interact within cytosol and extracellular fluid to drive metabolism, transport, and homeostasis.
  • Recurrent theme: “shape dictates function”—cell and molecular shapes depend on interactions like hydration, hydrogen bonding, and osmotic balance.
  • The material builds toward more advanced topics in physiology and cellular biology (e.g., osmoregulation, pH homeostasis, and fluid balance).

Quick Review Checklist for Exam

  • Define hydrolysis and dehydration synthesis; write their general forms.
  • Distinguish cytoplasm vs cytosol and extracellular fluid vs interstitial fluid.
  • Explain water’s polarity, hydrogen bonding, and the concept of hydration shells.
  • Describe what dissolves in water (solvent vs solute) and the principle of “like dissolves like.”
  • Differentiate hydrophilic vs hydrophobic substances with examples.
  • List water’s key physical properties and their biological relevance (high heat capacity, high heat of vaporization, cohesion, adhesion, surface tension).
  • Explain how phase changes work in water and why ice floats.
  • Define colligative properties and give an example (e.g., antifreeze).
  • Preview pH and the meaning of [H⁺] on the pH scale.