1.3: INTRODUCTION TO MACROMOLECULES

Building and Breaking Polymers

Biological macromolecules are built from small repeating subunits called monomers, which link together to form long chains called polymers. Two complementary chemical reactions govern how cells assemble and disassemble these polymers: dehydration synthesis (for building) and hydrolysis (for breaking down).

Dehydration synthesis [a reaction that removes water to form a covalent bond] joins two monomers into a larger molecule. During this reaction, a hydrogen ion (--H) is removed from one monomer and a hydroxyl group (--OH) is removed from the other. The loss of these components is equivalent to the removal of one water molecule (H₂O) from the reactants. A new covalent bond forms between the two monomers where the --H and --OH were removed. When this process repeats many times, linking monomer after monomer into a long chain, it is called polymerization. Dehydration synthesis therefore requires energy input and produces water as a byproduct.

Hydrolysis [a reaction that uses water to break a covalent bond] is the reverse process. When water is added across the covalent bond that links two monomers in a polymer, that bond is cleaved. Specifically, the hydrogen ion from the water molecule attaches to one monomer, and the hydroxyl group attaches to the other. This restores each monomer to its original, unlinked form. Hydrolysis breaks large molecules into smaller molecules and is essential for digestion and recycling of biological materials.

Because dehydration synthesis and hydrolysis are essentially mirror images, they share the same participants — monomers, polymers, water, and covalent bonds — but run in opposite directions. Cells use dehydration synthesis when they need to store energy or build structural components, and they use hydrolysis when they need to release monomers for energy or new construction.

MisconceptionStudents often think hydrolysis simply "adds water" to a molecule without changing bonds. In reality, the water molecule is split — its H goes to one monomer and its OH goes to the other — and a covalent bond is broken in the process.

Exam TipAlways mention that a covalent bond is broken (hydrolysis) or formed (dehydration synthesis); stating only "water is added" or "water is removed" is incomplete.

Examiner InsightAP free-response questions frequently ask students to compare these two reactions. A strong answer explicitly names both the bond change (formed vs. broken) and the role of water (released vs. consumed) for each reaction.

Exam TipUse the table format mentally — describe the direction, what happens to water, and what happens to the covalent bond for full credit.

Exam TipLabel the covalent bond between the joined monomers and annotate where the H and OH from water attach during hydrolysis.