In a polypeptide or a protein, amino acids are linked by a peptide bond which is formed when the carboxyl (-COOH) group of one amino acid reacts with amino (-NH2) group of the next amino acid with the elimination of a water moiety. Unsaturated fatty acids are with one or more C = C double bonds. In nucleic acids, a phosphate moiety links the 3'-carbon of one sugar of one nucleotide to the 5'-carbon of the sugar of the succeeding nucleotide. The bond between the phosphate and hydroxyl group is an ester bond. As there is one such ester bond on either side, it is called phosphodiester bond. In a polysaccharide, the individual monosaccharides are linked by a glycosidic bond.