Topic 1.3 Notes – Introduction to Macromolecules

1. Monomers, Polymers, and Macromolecules

Life builds complexity from small parts.

• Monomer = a small molecular building block
• Polymer = a large molecule made of many repeating monomers joined by covalent bonds
• Polymerization = the process of linking monomers together

Those covalent bonds are strong intramolecular bonds. They create the backbone of biological macromolecules.

The Four Biological Macromolecules

Each major macromolecule has characteristic building blocks and functions.

Lipids are different. They are large molecules, but most are not true polymers because they are not made of long repeating chains of identical monomers.

Keep this pattern in your head: small subunits → covalent bonds → large functional molecules.

2. Covalent Bonds and Molecular Interactions

Polymers are held together by covalent bonds, where atoms share electrons.

Types of Covalent Bonds

• Nonpolar covalent
  • Electrons shared equally
  • No partial charges
  • Common in C-C and C-H bonds
  • Contributes to hydrophobic behavior
• Polar covalent
  • Electrons shared unequally
  • Creates partial charges (δ⁺ and δ⁻)
  • Due to differences in electronegativity
  • Makes molecules polar and often water-soluble

Inside a molecule, these are intramolecular forces.

Between molecules, weaker attractions matter too.

Hydrogen Bonds

• Attraction between a hydrogen bonded to N, O, or F and another electronegative atom
• Weak compared to covalent bonds
• Crucial for:
  • DNA base pairing
  • Protein folding
  • Properties of water

Hydrogen bonds are intermolecular, not covalent. That distinction shows up in multiple-choice questions.

Covalent bonds build the structure. Hydrogen bonds stabilize shape.

3. Dehydration Synthesis Builds Polymers

Dehydration synthesis is how cells connect monomers.

Two monomers join through a covalent bond. During the reaction:

• One monomer loses a hydrogen (H⁺)
• The other loses a hydroxyl group (OH⁻)
• These combine to form H₂O
• A new covalent bond forms between the monomers

Water is removed, which is why it’s called “dehydration.” In the top half of the diagram below, notice how an OH from one monomer and an H from the other combine to form water, leaving the two monomers linked by a covalent bond.

Dehydration synthesis and hydrolysis overview

Important characteristics:

• Builds larger molecules from smaller ones
• Forms polymers (polymerization)
• Requires enzymes in cells
• Requires energy input (endergonic)
• An anabolic process (increases complexity)

Examples:

• Amino acids joining to form a polypeptide
• Glucose molecules linking to form starch
• Nucleotides forming DNA

If a question describes water being produced while a larger molecule forms, that’s dehydration synthesis.

4. Hydrolysis Breaks Polymers

Hydrolysis is the reverse reaction.

Water is added to break a covalent bond between monomers.

• A water molecule splits into H⁺ and OH⁻
• H attaches to one monomer
• OH attaches to the other
• The covalent bond is cleaved

Hydrolysis reaction breaking a polymer

In the diagram, a water molecule is added to the chain, splitting one bond so that one fragment gains an H and the other gains an OH.

Key features:

• Breaks polymers into monomers
• Uses water
• Releases energy (exergonic overall in biological systems)
• Requires enzymes
• A catabolic process (reduces complexity)

This is what happens during digestion. Proteins, carbohydrates, and nucleic acids in food are hydrolyzed into their monomers so your cells can reuse them.

If a passage mentions water being consumed while a bond breaks, think hydrolysis.

5. How These Reactions Support Biological Systems

Cells constantly balance building and breaking.

• Dehydration synthesis builds macromolecules for growth and repair.
• Hydrolysis recycles molecules and releases usable components.
• Enzymes regulate both processes to maintain homeostasis.

This connects to a core AP idea: biological systems depend on regulated molecular interactions. Change the structure of a molecule and you change how it behaves in the whole system.

Even a single broken bond can alter shape, which can change function, which can affect an entire pathway.