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1.3 Introduction to Macromolecules

Syllabus
2025
Topic
1.3
Level
—

Learning objectives

1.3A—Describe the chemical reactions that build and break biological macromoleculesDescribe the chemical reactions that build and break biological macromolecules.• Hydrolysis is a chemical reaction involving the cleaving of covalent bonds. This type of reaction breaks down molecules into smaller molecules. When water is added to the bond between monomers in a polymer, the bond is broken. The hydrogen ion from a water molecule is added to one monomer and the hydroxyl group of the water molecule is added to the other monomer, completing the reaction.• Dehydration synthesis occurs when two smaller molecules are joined together through covalent bonding. A hydrogen ion is removed from one monomer and a hydroxyl group is removed from the other. This causes the loss of the equivalent of a water molecule from the reactants and the connection of the two remaining monomers. The connection of many monomers is known as polymerization.

Build by Removing Water; Break by Adding Water

Feature Dehydration synthesis Hydrolysis
Main result Joins two smaller molecules Splits a larger molecule into smaller molecules
Covalent bond Formed between the smaller molecules Cleaved between monomer units
Water Equivalent of one water molecule is removed from reactants One water molecule is added at the bond
Atom placement H is removed from one monomer and OH from the other H is added to one product and OH to the other
Effect on polymer Repetition connects many monomers in polymerization Repetition breaks a polymer into smaller units

For dehydration synthesis, identify two smaller molecules, remove H from one and OH from the other, then form a covalent bond between the remaining parts. The removed H and OH are equivalent to H₂O. Repeating this joining reaction produces a polymer.

For hydrolysis, place water across the covalent bond to be cleaved. The bond breaks; H from water completes one smaller product and OH completes the other. The products contain the atoms of the original molecule plus the atoms supplied by water.

The names describe reaction chemistry, not simply whether water is nearby. Dehydration synthesis forms a covalent bond while losing the equivalent of water; hydrolysis consumes water while cleaving a covalent bond.