The correct answer is A: side groups. The differences between amino acids are primarily in their side groups, also known as R-groups. Amino acids consist of a central carbon atom (alpha carbon) bonded to an amino group, a carboxyl group, a hydrogen atom, and an R-group. The R-group, which differs for each amino acid, determines the unique chemical properties of that particular amino acid. It can be a simple hydrogen atom (in the case of glycine), a methyl group (in alanine), a hydroxyl group (in serine), or any other functional group. These variations in the side groups give each amino acid its distinct physical and chemical properties, allowing them to perform different roles in biological processes.
