The two functional groups always found in amino acids are carboxyl and amino groups. Both groups are acidic. A peptide bond occurs when the carboxyl group of one amino acid joins the amino end of another.
A carboxyl group consists of one carbon atom, two oxygen atoms and one hydrogen atom. The amino group consists of one nitrogen atom and two hydrogen atoms. The carboxyl and amino groups lie at opposite ends of an amino acid.
Two amino acids form a peptide, or amide, bond when they join. The amino and carboxyl ends of the two amino acids come together, and the nitrogen of the amino group bonds with the carbon on the carboxyl group. The hydrogens on the amino group and an oxygen on the carboxyl group combine to form a molecule of water.