There are 20 different amino acids that are commonly found in proteins, and they are linked together in long chains to form protein molecules.

Each amino acid has a central carbon atom bonded to:

* An amino group (-NH2)

* A carboxyl group (-COOH)

* A hydrogen atom (-H)

* A side chain (R group)

The R group is what makes each amino acid unique and determines its properties. The sequence of amino acids in a protein chain is called the primary structure of the protein, and it determines the protein's three-dimensional shape and function.