Consider the following statements about the denaturation of proteins.
(A) During denaturation, the secondary and tertiary structures of a protein are destroyed but its primary structure remains intact.
(B) A change in temperature or pH disturbs the hydrogen bonds of a native protein.
(C) Curdling of milk is caused by the formation of acetic acid by the bacteria present in milk.
(D) The coagulation of egg white on boiling is an example of denaturation.
Which of the statements given above are correct? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (A), (B) and (D) only
A native protein has its own three-dimensional shape and biological activity. In denaturation, a physical or chemical change disturbs the hydrogen bonds that hold this shape, so the protein unfolds and loses its activity.
(A) During denaturation, the secondary and tertiary structures of a protein are destroyed but its primary structure remains intact. — True. The folding is lost, but the sequence of amino acids (the primary structure) is not changed.
(B) A change in temperature or pH disturbs the hydrogen bonds of a native protein. — True. These are the physical and chemical changes that start denaturation.
(C) Curdling of milk is caused by the formation of acetic acid by the bacteria present in milk. — False. The bacteria in milk form lactic acid, not acetic acid; the acid denatures the milk proteins.
(D) The coagulation of egg white on boiling is an example of denaturation. — True. Heat denatures the egg-white proteins, which is why they set into a solid.
NCERT (2026-27 edition):
10.2.4 Denaturation of Proteins – “…When a protein in its native form, is subjected to physical change like change in temperature or chemical change like change in pH, the hydrogen bonds are disturbed. Due to this, globules unfold and helix get uncoiled and protein loses its biological activity. This is called denaturation of protein. 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. Another example is curdling of milk which is caused due to the formation of lactic acid by the bacteria present in milk.”