Concept:Amino acids are organic compounds that serve as building blocks of proteins, each containing a central carbon bonded to an amino group, a carboxyl group, a hydrogen atom, and a variable side chain (R group).
Key Fact:Amino acids are indeed substituted methanes because the central carbon is bonded to four different groups; serine is a polar but not aromatic amino acid, valine is neutral (non‑polar), and lysine is basic (not acidic).
Explanation:Statement A is correct: All amino acids have a tetrahedral carbon atom attached to an amino group, a carboxyl group, a hydrogen, and an R group – this makes them substituted methanes.
Statement B is incorrect:
Serine has a hydroxyl group in its side chain, making it an alcoholic (polar) amino acid, not aromatic.
Statement C is correct:
Valine has a non‑polar aliphatic side chain and is classified as a neutral amino acid.
Statement D is incorrect:
Lysine contains an additional amino group in its side chain, so it is a basic (positively charged) amino acid, not acidic.
Answer:Option C (A and C only).
