Final answer:
Serine and threonine are classified as uncharged polar amino acids due to their hydroxyl group in the side chain. They are involved in phosphorylation in signaling pathways.
Step-by-step explanation:
Serine and threonine are classified as uncharged polar amino acids. These amino acids have polar side chains that contain hydroxyl groups (-OH), which makes them hydrophilic or water-attracting. Serine and threonine are known for their involvement in phosphorylation, as their hydroxyl groups can be phosphorylated in signaling pathways.