Introduction to Biological Macromolecules

Almost every large molecule in a cell is built the same way: from small repeating units strung together into long chains. The small unit is a monomer; the chain built from many of them is a polymer. The individual monomers in a chain are its subunits — a polymer is not a new kind of matter, it is its subunits joined in sequence, and the identity and order of those subunits is what makes one polymer different from another.

Cells join monomers by dehydration synthesis: forming the bond removes a molecule of water — one monomer gives up an —OH, its neighbor gives up an —H, and together those make the water that leaves. The reverse, hydrolysis, adds water back to break a bond, splitting the polymer into its monomers. Synthesis costs energy and expels water; digestion consumes water and releases it. Same bond, opposite directions.

Four classes of macromolecule run a cell, and three of them are polymers: carbohydrates are polymers of monosaccharides, proteins are polymers of amino acids, and nucleic acids are polymers of nucleotides. The fourth class, lipids, is the exception — a lipid is not a polymer, because its parts (a glycerol and its fatty acids) are not repeating identical subunits linked into a chain. Grouping lipids with the polymers is one of the most common slips in this topic.

Why the sequence of subunits matters is the whole point: structure predicts function. Twenty amino acids can be ordered into countless proteins, and the order fixes how the chain folds, which fixes what the protein can do. Read a monomer's identity or a chain's arrangement and you can predict the molecule's behavior — the same structure-function logic that runs through all of biology, now at the scale of a single chain.