Which of the following amino acids is classified under polar amino acids?

Serine is categorized as a polar amino acid due to the presence of its hydroxyl (-OH) group in its side chain. This functional group allows serine to form hydrogen bonds with water and other polar molecules, contributing to its solubility in aqueous environments. Polar amino acids are characterized by their ability to interact favorably with water, and serine’s structure makes it a prime example of this property.

In contrast, proline has a unique cyclic structure that renders it less polar than serine. Arginine, although it has a polar side chain that can interact with water, is better classified under basic amino acids due to its guanidinium group. Phenylalanine, being an aromatic amino acid with a hydrophobic side chain, lacks the polar characteristics necessary for classification in this group. Thus, serine’s hydroxyl group is what firmly establishes its classification as a polar amino acid.