Serine is one of the polar amino acids. Its structure includes:
- An amino group \( -\text{NH}_2 \)
- A carboxylic group \( -\text{COOH} \)
- And a side chain containing a hydroxyl group \( -\text{OH} \)
Structure of serine:
\[
\text{HO-CH}_2\text{-CH(NH}_2\text{)-COOH}
\]
Hence, the additional functional group is \( -\text{OH} \).