An amino acid is a monomer, which means it's a small molecule that can join with other similar molecules to form a larger structure. Specifically, amino acids are the monomers of proteins.

Here's why:

* Proteins are large, complex molecules that perform a wide range of functions in living organisms.

* Amino acids are the building blocks of proteins. They link together in long chains to form these complex structures.

So, while amino acids are themselves biological molecules, their primary function is to serve as the building blocks of proteins.