Chemical Bonds, Water Properties, and Biological Macromolecules

Posted on Mar 14, 2026 in Biochemistry

Ionic Bonds

Mechanism

One atom loses electrons while another gains electrons, creating ions (positive cations and negative anions). Opposite charges attract, forming a bond. These are relatively weak in water because water is polar and surrounds the charged ions, causing dissociation.

Covalent Bonds

Mechanism

Atoms share electrons, typically between two non-metals. These are much stronger than ionic bonds.

Types of Covalent Bonds

  • Nonpolar covalent: Equal sharing of electrons with no partial charge.
  • Polar covalent: Unequal sharing of electrons, creating partial positive (δ⁺) and partial negative (δ⁻) charges. Water is a polar covalent molecule.

Hydrogen Bonds

Definition

A weak attraction between a δ⁺ hydrogen atom and a δ⁻ electronegative atom (Oxygen or Nitrogen). In water, hydrogen from one molecule bonds to the oxygen of another.

Biological Importance

  • Creates cohesion and surface tension.
  • Stabilizes protein structure.
  • Holds DNA strands together.

Hydrogen bonds are weak individually, but powerful in large numbers.

Water Properties

Oxygen is more electronegative, making it δ⁻ and hydrogen δ⁺, which allows for hydrogen bonding.

Key Properties

  • Cohesion: Hydrogen bonds between water molecules.
  • Adhesion: Hydrogen bonds to other polar molecules.
  • Surface Tension: Cohesion at the surface.
  • High Specific Heat: Hydrogen bonds absorb significant heat.
  • High Heat of Vaporization: Energy required to break hydrogen bonds.
  • Universal Solvent: Polarity dissolves ions and polar substances.

Carbon

Carbon has 4 valence electrons, allowing it to form 4 covalent bonds. It creates chains and rings, serving as the backbone of all macromolecules.

Chemical Reactions

  • Dehydration Synthesis: Removes H₂O to build polymers (Monomer + Monomer → Polymer + H₂O).
  • Hydrolysis: Adds H₂O to break polymers into monomers.

Macromolecules

1. Carbohydrates

  • Elements: C, H, O (1:2:1 ratio).
  • Monomer: Monosaccharide.
  • Bond: Glycosidic.
  • Polymers: Starch, Glycogen, Cellulose.
  • Function: Quick energy and structural support.

2. Lipids

  • Elements: C, H, O.
  • Subunits: Glycerol and fatty acids.
  • Types:
    • Triglyceride: 3 fatty acids + glycerol (long-term energy).
    • Phospholipid: 2 fatty acids + glycerol + phosphate (amphipathic; cell membranes).
    • Steroid: 4 fused rings (hormones).

3. Proteins

  • Elements: C, H, O, N (sometimes S).
  • Monomer: Amino acid (Amino group, Carboxyl group, R group).
  • Bond: Peptide.
  • Levels of Structure: Primary (sequence), Secondary (α-helix/β-sheet), Tertiary (3D folding), Quaternary (multiple chains).
  • Enzymes: Biological catalysts that lower activation energy. Factors like temperature and pH can cause denaturation.

4. Nucleic Acids

  • Elements: C, H, O, N, P.
  • Monomer: Nucleotide (Phosphate, Sugar, Nitrogen base).
  • DNA: Double-stranded, Deoxyribose, A–T and C–G.
  • RNA: Single-stranded, Ribose, A–U and C–G.
  • Bonds: Phosphodiester (backbone) and Hydrogen bonds (between bases).