Imagine you are sorting the contents of a living cell by size. Most of the substances you find are small — little molecules that dissolve easily and float freely, like grains of sand. But mixed in among them are a few giants, molecules so large they stand in a class of their own. Those giants are the macromolecules.
A macromolecule is a very large biological molecule with a high molecular weight, generally in the region of ten thousand daltons and above. When the substances of a tissue are separated, the macromolecules turn up in what is called the acid-insoluble fraction — the part that does not dissolve away with the small molecules. Only a few kinds of true macromolecule are found there: proteins, nucleic acids and polysaccharides.
What makes these three qualify as macromolecules is not just their size but the way they are built. Each one is a polymer — a long molecule assembled by stringing together many small repeating building blocks. Proteins are long chains of amino acids, nucleic acids (DNA and RNA) are long chains of nucleotides, and polysaccharides are long chains of sugars. Joining thousands of small units end to end is exactly how nature reaches such enormous molecular weights, and it is why macromolecules are also called biomacromolecules.
This matters because the biggest, most important jobs in a cell are done by these large molecules. Nucleic acids carry the genetic information; polysaccharides build supporting structures and store energy; proteins act as enzymes, hormones, antibodies and structural parts. The small molecules are the raw material, but the macromolecules are the machinery. …