Biomolecules

Proteins are polymers of amino acids, and the way these monomers connect is central to understanding protein structure. When two amino acids combine, the carboxyl group of one reacts with the amino group of the next, releasing a molecule of water and forming a peptide bond, as explained in NCERT Class 11, Chapter 9 (Biomolecules); this dehydration or condensation reaction links amino acids into long polypeptide chains. The glycosidic bond is incorrect because it joins monosaccharide units to build carbohydrates such as starch and cellulose, not amino acids. The phosphodiester bond is wrong here, since it connects nucleotides through sugar and phosphate groups to form the backbone of nucleic acids. The ester bond describes the linkage between glycerol and fatty acids in fats and oils, again unrelated to protein assembly. A chemical consistency check supports the answer: only a bond formed between a carboxyl and an amino group, accompanied by loss of water, can correctly join two amino acids, and that linkage is the peptide bond.