Proline has a nonpolar aliphatic side chain, while serine has an uncharged polar side chain.

### Why this is right Proline’s side chain is largely hydrocarbon and loops back to its amino nitrogen, making it nonpolar and structurally distinctive. Serine has a hydroxyl group that can interact with water, so its side chain is uncharged but polar. ### The concept Amino-acid categories are based chiefly on side-chain properties. The categories help predict solubility, protein folding, and interactions, but do not replace examining the actual side-chain structure. ### Terms - **Aliphatic**: describing a non-aromatic carbon structure. - **Polar**: having an uneven distribution of charge that can support interactions with water. ### Remember Serine’s hydroxyl makes it polar; proline’s hydrocarbon-rich ring makes it nonpolar.