Building and breaking polymers

Some molecules in your cells contain thousands or even millions of atoms. Cells don’t assemble these giants one atom at a time. They link ready-made small pieces into chains, much as you’d build a long chain out of paper clips.

Monomers, polymers, and macromolecules

A small molecule that serves as a building block is a monomer (“one part”). A large molecule made of many monomers linked together is a polymer (“many parts”).

A large molecule assembled from many smaller building blocks joined by covalent bonds is a macromolecule. Despite the name, there’s no size cutoff: what counts is that the molecule is built from many covalently linked pieces. Proteins, nucleic acids (DNA and RNA), and large carbohydrates are all macromolecules, and a single strand of DNA can contain millions of building blocks.

Dehydration reactions build polymers

Cells grow a polymer by adding one monomer at a time, and each addition releases a molecule of water.

In a condensation reaction, two molecules join through a covalent bond, usually giving off a small molecule in the process. When that small molecule is water, the reaction is also called a dehydration reaction. One molecule gives up a hydroxyl group (–OH) and the other gives up a hydrogen atom (–H). These leave together as H₂O, and the two molecules bond to each other where those atoms were removed.

Each dehydration reaction adds one monomer, so repeating the reaction grows a chain as long as the cell needs.

Hydrolysis reactions break polymers apart

Breaking a polymer down reverses the process. In a hydrolysis reaction, a water molecule is split across the bond between two monomers: the H goes to one monomer, the –OH goes to the other, and one monomer is released. Every bond broken this way uses up one water molecule.

Cells hydrolyze polymers whenever they need the monomers. A plant stores energy as starch, a long chain of sugar units. When the plant needs that energy, it hydrolyzes the starch back into individual sugars. Your digestive system does the same thing to the starch in a slice of bread.

The names describe what happens to the water. Dehydration means water is removed as a bond forms. Hydrolysis means “splitting with water”: a water molecule is used up to split a bond.

Four classes of organic molecules

Every living thing, from bacteria to oak trees, contains the same four classes of organic molecules.

Not all of these are macromolecules. Proteins and nucleic acids always are. Carbohydrates count only when they’re large: table sugar (sucrose) contains just two sugar units, while a polysaccharide may contain thousands.

Lipids never count, because they aren’t chains of many covalently linked monomers. Lipids can gather into large structures such as membranes, but the molecules in those structures aren’t covalently bonded to one another.

Check yourselfSome fat molecules are fairly large. Why aren’t they classified as macromolecules?

A macromolecule is defined by how it’s built, not by its size: it’s made of many smaller building blocks covalently linked together. A fat is a single glycerol joined to three fatty acids, not a long chain of repeating monomers, so it doesn’t qualify.