The differences between amino acids are primarily found in their side groups. Each amino acid has a central carbon atom, also known as the alpha carbon, which is bonded to four different chemical groups: an amino group (NH2), a carboxyl group (COOH), a hydrogen atom (H), and a specific side group or R-group. It is this side group that distinguishes one amino acid from another. The R-group can vary in size, shape, and chemical properties, which leads to differences in the functional and structural characteristics of amino acids. On the other hand, while hydrogen atoms are present in all amino acids, they do not contribute to the primary differences between them. Therefore, the correct answer is A: side groups.
