Final answer:
Amino acids differ from one another due to their unique R-groups, which determine their chemical nature and influence protein structure and function.
Step-by-step explanation:
In general, what makes one kind of amino acid different from another is the presence of a unique side chain, or R-group, attached to the α-carbon. While every amino acid has a central carbon atom bonded to an amino group (NH2), a carboxyl group (COOH), and a hydrogen atom, the R-group varies among the 20 proteinogenic amino acids. These R-groups can range from simple hydrogen atoms to complex ring structures and can be nonpolar, polar, charged, or aromatic, which determines the chemical nature of each amino acid. For example, amino acids with nonpolar R-groups, like valine and alanine, are hydrophobic, whereas those with polar or charged R-groups, such as lysine or arginine, are hydrophilic. This diversity in side chains influences not just the amino acid's reactivity and interactions with other molecules but also the folding, structure, and function of the proteins they form.