What is a protein?

• A. A chain of amino acids bonded by peptide bonds
• B. A chain of nucleotides bonded by peptide bonds
• C. A lipid molecule
• D. A type of carbohydrate

Answer: (A)

Proteins are polymers built from amino acids. Each amino acid is linked to the next by a peptide bond, formed when the amino group of one amino acid and the carboxyl group of the next react. The sequence of amino acids—the primary structure—determines how the chain folds into a three-dimensional shape, which in turn defines the protein’s function in the body. This description—an chain of amino acids bonded by peptide bonds—captures what proteins are and how they’re formed. The other options describe different biomolecules: nucleic acids are made of nucleotides connected by bonds between sugar and phosphate groups, lipids are not formed from amino acids, and carbohydrates are sugar-based polymers.