Read the passage carefully and answer the questions.
Carbohydrates are naturally formed organic compounds from various plants. The common chemical formula, \( (C_x(H_2O)_y) \) was considered as hydrates of carbon to derive the various carbohydrates. For instance, the general formula for glucose \( (C_6H_{12}O_6) \) easily fits into the above formula. Carbohydrates also called saccharides are classified on the basis of their behavior towards hydrolysis. Monosaccharides cannot be hydrolyzed further to simpler units of polyhydroxy aldehyde or ketones. About 20 monosaccharides are known to occur in nature. Carbohydrates that yield large number of monosaccharide units are called polysacccharides. Amino acids contain \( (-NH_2) \) and \( (-COOH) \), which can be classified in various forms. Proteins are polymers of alpha amino acids which are connected to each other by a peptide bond or peptide linkage. When a native protein is subjected to physical change, its globules unfold and helix get uncoiled and the protein loses its biological activity. The protein is said to be denaturated.
The peptide linkage formed between the reaction of glycine and alanine is represented as
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → -CO-NH-
A peptide linkage is an amide bond. It forms when the –COOH group of one amino acid reacts with the –\( \ce{NH2} \) group of another amino acid, and a molecule of water is eliminated.
In glycylalanine (Gly-Ala) the group that joins the two residues is –CO–NH–, the peptide bond.
Why the other options are incorrect
Option (2) -C-O-C- — Incorrect. An oxygen bridge between two carbon atoms is an ether-type link, such as the glycosidic linkage of carbohydrates; amino acids are not joined this way.
Option (3) -C-N-O- — Incorrect. No bond from nitrogen to oxygen is formed when two amino acids combine; the nitrogen bonds to the carbonyl carbon.
Option (4) -C-O-P- — Incorrect. A carbon–oxygen–phosphorus link is a phosphoester link, found in the sugar-phosphate backbone of nucleic acids, not in proteins.
NCERT (2026-27 edition):
Structure of Proteins
