Final answer:
The variable part of an amino acid is the side chain or R-group, which determines its unique properties and influences the structure and function of proteins.
Step-by-step explanation:
The variable side chain of an amino acid is the part that is responsible for determining the unique properties of that amino acid. The side chain, also known as the R-group, can vary in size, shape, solubility, and ionization properties. Different amino acids have different side chains, which give them distinct chemical and physical characteristics. For example, some side chains are nonpolar, while others are polar or acidic. These variations in side chains contribute to the diverse functions and structures of proteins.