Final answer:
Each amino acid is differentiated by its side chain, or R group, which is attached to the central alpha carbon and is unique for each amino acid.
Step-by-step explanation:
The functional group that makes each amino acid different in structure is the side chain, or R group.
This R group varies from one amino acid to another, giving each one its unique properties.
While the amino group (NH₂), carboxyl group (COOH), and a hydrogen atom are consistent in all amino acids, it's the R group attached to the central carbon, also known as the alpha (α-) carbon, that distinguishes one amino acid from the next.