Key Concepts: Covalent Bonds, Water, Hydrophilic/Hydrophobic, Ionic Bonds, Macromolecules

Exam overview

  • Exam Zero: multiple choice; focused mainly on biochemistry

  • Unit One: multiple choice over all of Unit 1

  • LA Study Center: schedule on Canvas; any LA can help; LAs may post study tips and answer questions in discussion forums

Electronegativity, polarity, and bond types

  • Electronegativity: tendency of an atom to attract electrons

  • Covalent bonds: sharing of electrons

    • Nonpolar covalent: electrons shared equally

    • Polar covalent: electrons shared unequally

  • Polarity criterion (rule of thumb):
    |
    \Delta \chi\,| > 0.4
    ⇒ textpolarbond\Rightarrow\ text{ polar bond}

  • Partial charges in polar bonds: the more electronegative atom gains partial negative charge (δ−\delta^-); the other atom gains partial positive charge (δ+\delta^+)

  • Water as a key example: H2O\mathrm{H_2O} is polar; O is more electronegative than H, creating partial charges

    • Hydrogen atoms carry δ+\delta^+; oxygen carries δ−\delta^-

  • Water molecule interactions:

    • Covalent bonds within water: polar covalent

    • Intermolecular interactions: hydrogen bonds (weak attractions, not true covalent bonds)

    • Hydrogen bond example: between a hydrogen (attached to an electronegative atom) and another electronegative atom with partial negative charge (e.g., another water molecule)

  • Hydrogen bonding importance: contributes to DNA structure and various cellular processes

Hydrogen bonds and water structure

  • Hydrogen bond: involves hydrogen and another atom with a partial negative charge; typically O or N

  • Hydrogen bonds are weaker than covalent bonds but crucial for molecular interactions and structure

  • Boiling water: hydrogen bonds break more easily than covalent bonds

Hydrophilic vs hydrophobic

  • Hydrophilic: water-loving; polar molecules that can form hydrogen bonds with water; mix with water

  • Hydrophobic: water-fearing; nonpolar molecules that do not form favorable interactions with water; Insoluble in water (e.g., oil)

  • Relationship: polar => hydrophilic; nonpolar => hydrophobic

  • Exceptions: some molecules can be nonpolar and hydrophobic due to molecular symmetry that cancels dipoles

Ionic bonds (brief for this unit)

  • Ionic bond: complete transfer of electrons between atoms; strong electrostatic attraction between oppositely charged ions

  • Formed when differences in electronegativity are large (often much greater than 0.4; typically around 2+ in common salts like NaCl)

  • Example: NaCl\mathrm{NaCl}; Na becomes positive, Cl becomes negative

Major biological macromolecules

  • Four major groups found in life:

    • Carbohydrates

    • Proteins

    • Nucleic acids

    • Lipids

  • Monomers that build these macromolecules:

    • Carbohydrates: monosaccharides

    • Proteins: amino acids

    • Nucleic acids: nucleotides

    • Lipids: monomers not always defined in the same way (less emphasis here)

Quick study/participation reminders

  • Work with peers; practice drawing electron pairs and identifying partial charges on molecules

  • Determine polar vs nonpolar using electronegativity differences

  • Be prepared to explain why water is central to biological interactions

  • If you have technical issues with Top Hat or activities, see the instructor before the end of class