The building blocks of proteins are amino acids. Which molecule is NOT an amino acid?

• A. Glucose
• B. Glycine
• C. Leucine
• D. Valine

Answer: (A)

Amino acids are defined by having both an amino group and a carboxyl group attached to the same carbon, plus a distinctive side chain. This setup lets them link together via peptide bonds to form proteins. Glucose lacks these features; it is a carbohydrate—a monosaccharide with a carbonyl group and multiple hydroxyl groups, not an amino acid. Glycine, leucine, and valine all fit the definition of amino acids because they contain both the amino and carboxyl groups and a side chain. Therefore glucose is not an amino acid.