Answer the question on the basis of passage given below:
Proteins are the polymers of about twenty different \(\alpha \)-amino acids which are linked by peptide bonds. Proteins perform various structural and dynamic functions in the organisms. Proteins which contain only \(\alpha \)-amino acids are called simple proteins. The secondary or tertiary structure of proteins gets disturbed on change of pH or temperature and they are not able to perform their functions. This is called denaturation of proteins. Enzymes are biocatalysts which speed up the reactions in biosystems. Vitamins are accessory food factors required in the diet. They are classified as fat soluble (A, D, E and K) and water soluble (B group and C). Deficiency of vitamins leads to many diseases. Nucleic acids are the polymers of nucleotides which in turn consist of a base, a pentose sugar and phosphate moiety. Nucleic acids are responsible for the transfer of characters from parents to offspring. There are two types of nucleic acids – DNA and RNA. DNA contains a five carbon sugar molecule called 2-deoxyribose whereas RNA contains ribose.
During denaturation of proteins
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → \(1^o\) structure remains unaltered
A protein has four levels of structure: the primary structure is the sequence of amino acids joined by peptide bonds; the secondary (α-helix, β-pleated sheet) and tertiary structures are the folding held by hydrogen bonds, disulphide links, and van der Waals and electrostatic forces; the quaternary structure is the arrangement of subunits.
On a change of temperature or pH, the hydrogen bonds and other weak forces break. The protein unfolds, its globules uncoil and it loses its biological activity: this is denaturation. The covalent peptide bonds are not broken, so the amino acid sequence stays the same.
Why the other options are incorrect
Option (1) Biological activity remains unaltered — Incorrect. The protein loses its biological activity on denaturation, because its activity depends on its shape.
Option (2) \(3^o\) structure remains unaltered — Incorrect. The tertiary structure is destroyed; it is held by weak forces that break.
Option (3) \(2^o\) structure remains unaltered — Incorrect. The helices and sheets are held by hydrogen bonds, which break, so the secondary structure is lost too.
NCERT (2026-27 edition):
Denaturation of Proteins – “During denaturation secondary and tertiary structures are destroyed but primary structure remains intact. The coagulation of egg white on boiling is a common example of denaturation.”