Final answer:
The R group, or side chain, varies among different amino acids and is responsible for their unique characteristics that affect protein structure and function.
Step-by-step explanation:
The group that varies among different amino acids is the R group (side chain). Each amino acid is composed of an amino group, a carboxyl group, and an R group attached to the α-carbon. It is the R group that gives each of the 20 amino acids its unique characteristics, influencing factors such as size, shape, solubility, and ionization properties, which in turn, affect protein structure and function.
The amino and carboxyl groups are common to all amino acids and do not vary. The R group, however, can range from a single hydrogen atom, as in glycine, to complex structures with functional groups, as seen in other amino acids such as glutamic acid. This diversity in R groups allows proteins to exhibit a wide range of chemical behaviors and biological activities.