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.
During denaturation process of protein which structure(s) remain intact?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Primary structure
Proteins are described at four levels:
Primary structure: the sequence of amino acids, held by covalent peptide bonds.
Secondary structure: the α-helix or β-pleated sheet, held by hydrogen bonds between the >C=O and –NH groups of the chain.
Tertiary structure: the further folding of the chain into fibrous or globular shapes, held by hydrogen bonds, disulphide links, van der Waals and electrostatic forces.
Quaternary structure: the arrangement of two or more polypeptide subunits with respect to each other.
Denaturation breaks only the weak forces (mainly hydrogen bonds) that hold the folded shapes; the covalent peptide bonds are not broken. So the secondary and tertiary structures are lost, while the primary structure remains intact.
Why the other options are incorrect
Option (1) Secondary structure — Incorrect. The α-helix and β-pleated sheet are held by hydrogen bonds, which are the very bonds disturbed in denaturation.
Option (3) Tertiary structure — Incorrect. The folding of the chain depends on weak forces and is lost when the globules unfold.
Option (4) Quaternary Structure — Incorrect. The subunits are held together by the same kind of weak forces, so this arrangement is also lost.
NCERT (2026-27 edition):
Denaturation of Proteins
Note: The same/similar question has been asked in different years by NTA as per the details given below:
| 2026 | 2025 | 2024 | 2023 | 2022 |
|---|---|---|---|---|
| 26-05 S1 | 02-06 S1 26-05 S2 | – | – | – |