During the formation of a peptide bond, which of the following takes…

The correct answer is Option (4) → A hydroxyl group is lost from the carboxyl group of one amino acid and a hydrogen atom is lost from the amino group of another amino acid

• A peptide bond is an amide bond, –CO–NH–, formed between the –COOH group of one amino acid and the –NH₂ group of another, with the elimination of one water molecule.

• The –COOH group gives up its –OH and the –NH₂ group gives up one H; together they form \( \ce{H2O} \), and the C of the carbonyl joins the N. For glycine and alanine:

      \( \displaystyle \ce{H2N-CH2-COOH + H2N-CH(CH3)-COOH -> H2N-CH2-CO-NH-CH(CH3)-COOH + H2O} \)

  The product is the dipeptide glycylalanine.

Why the other options are incorrect

Option (1) A hydrogen atom is lost from the carboxyl group of one amino acid and a hydrogen atom is lost from the amino group of another amino acid — Incorrect. Two H atoms do not make a water molecule, and the carboxyl group would keep its –OH, so no –CO–NH– bond could form.

Option (2) A hydroxyl group is lost from the carboxyl group of one amino acid and a hydroxyl group is lost from the amino group of another amino acid — Incorrect. The amino group, –NH₂, has no hydroxyl group to lose.

Option (3) A hydrogen atom is lost from the carboxyl group of one amino acid and a hydroxyl group is lost from the amino group of another amino acid — Incorrect. This reverses the roles: the –NH₂ group has no –OH, and losing only H from –COOH would leave –COO, not the C=O of an amide.

NCERT (2026-27 edition):
10.2.3 Structure of Proteins – “… Chemically, peptide linkage is an amide formed between –COOH group and –NH₂ group. The reaction between two molecules of similar or different amino acids, proceeds through the combination of the amino group of one molecule with the carboxyl group of the other. This results in the elimination of a water molecule and formation of a peptide bond –CO–NH–.”