What is an amino acid?

• A. Monomer that makes up proteins; contains carboxyl and amino functional groups
• B. A type of sugar used for energy
• C. A fatty acid chain
• D. A nucleotide component

Answer: (A)

Amino acids are the building blocks of proteins. Each amino acid has a central carbon attached to an amino group (–NH2) and a carboxyl group (–COOH), along with a side chain called the R group that varies between different amino acids. These two functional groups—the amino and carboxyl groups—define amino acids and allow them to link together through peptide bonds to form protein chains. The side chain determines the specific properties of each amino acid, such as charge and hydrophobicity, which influence how proteins fold and function. At body pH, amino acids often exist as zwitterions, carrying both positive and negative charges, which helps them interact in aqueous environments. This matches the description of an amino acid as a monomer that makes up proteins and contains carboxyl and amino groups. The other options describe sugars, fatty acids, and nucleotide components, which belong to carbohydrates, lipids, and nucleic acids respectively.