1.3 Introduction to macromolecules

Summary

Biological macromolecules are polymers assembled from monomer subunits through dehydration synthesis (also called a condensation reaction), a reaction that removes a water molecule and forms a covalent bond between adjacent monomers. Repeating this reaction many times — polymerization — produces the large molecules cells need for structure, energy storage, and information.

Key Definition Hydrolysis consumes a water molecule to break the covalent bond between monomers, splitting the hydrogen ion onto one monomer and the hydroxyl group onto the other, thereby dismantling polymers into their building blocks.

Together, these two reactions form a reversible system that cells use to build up and break down macromolecules as needed.

Dehydration Synthesis

• Builds polymers by forming a covalent bond between monomers and releasing one water molecule per bond formed
• Dominates during growth and biosynthesis, when cells assemble new polymers

Hydrolysis

• Breaks polymers by adding (consuming) one water molecule per bond broken, reversing dehydration synthesis
• Dominates during digestion and molecular recycling, when cells break polymers down into monomers

Both reactions center on covalent bonds and water, but they operate in opposite directions — a symmetry that AP exam questions frequently test by asking students to compare the two side by side, specifying the bond changes and the precise fate of the water molecule.