CDS (I) 2024 General Knowledge · Question 65 of 120

Correct — B, proteins. Egg white is about nine-tenths water and almost all of the rest is protein, chiefly ovalbumin, dissolved as separate compact globules that let light through — which is why raw white is clear and runny. Heat breaks the hydrogen bonds holding each globule in its folded shape, the chains uncoil, and the exposed chains tangle with one another into a solid network that scatters light. NCERT gives this exact example, saying that the coagulation of egg white on boiling is a common example of denaturation. The word 'denaturation' in the stem also fixes the answer on its own, because denaturation is defined only for proteins: it is the loss of a protein's native three-dimensional shape and biological activity when heat or a change in pH disturbs its bonds, with the secondary and tertiary structure destroyed while the primary sequence survives.

• (a)fats — Egg white contains almost no fat — the fat is in the yolk — and fats do not denature. Heating a fat melts it or, at high temperature, breaks it down; it never sets into a white solid.
• (c)ribose sugar — Ribose is the five-carbon sugar of RNA and of nucleotides such as ATP. It is present only in traces in egg white and, being a simple sugar with no folded structure, has nothing to denature.
• (d)carbohydrates — Egg white has less than one per cent carbohydrate. Heating a sugar caramelises or browns it rather than setting it, and carbohydrates have no native three-dimensional fold to lose.

A protein working in a living system has a unique three-dimensional shape, and it is called a native protein. Denaturation is what happens when a physical change such as a rise in temperature, or a chemical change such as a shift in pH, disturbs the hydrogen bonds that hold that shape: the globules unfold, the helix uncoils and the protein loses its biological activity. The secondary and tertiary structures are destroyed, but the primary structure — the sequence of amino acids joined by peptide bonds — stays intact.

The give-away is the word 'denaturation' itself, which belongs to proteins alone; a student who knows that can answer without thinking about eggs at all. The visible change is worth understanding because it explains why denaturation is usually irreversible in the kitchen: once the unfolded chains have cross-linked into a mesh they cannot find their way back to the original fold, so a boiled egg never turns clear again. The other everyday example NCERT pairs with it is the curdling of milk, where the acid produced by bacteria, rather than heat, does the unfolding. The same principle explains why a high fever is dangerous and why enzymes stop working outside their temperature range.

• Denaturation is the loss of a protein's native three-dimensional structure and biological activity on heating or a change in pH.
• Secondary and tertiary structures are destroyed during denaturation; the primary structure remains intact.
• The coagulation of egg white on boiling is the textbook example of thermal denaturation.
• Curdling of milk, caused by lactic acid from bacteria, is denaturation brought about chemically rather than by heat.
• The main protein of egg white is ovalbumin; the white is roughly 90 per cent water and carries almost no fat.

• Choosing fats because eggs are thought of as a fatty food, when the fat sits in the yolk and the question is about the white.
• Treating 'denaturation' as a general word for any change on heating, when it applies only to proteins.
• Assuming denaturation breaks the peptide bonds — the primary structure survives.

Asked as an everyday-observation item where the technical term in the stem, 'denaturation', already narrows the answer to one class of biomolecule.

No directly related past PYQ was found.

• practice — not a real PYQ

  During the denaturation of a protein, which level of structure remains intact?

  • (a)Primary
  • (b)Secondary
  • (c)Tertiary
  • (d)Quaternary

  Answer(a) Primary — the peptide bonds of the amino-acid sequence are not broken; only the folded structures above it are lost.

• practice — not a real PYQ

  The curdling of milk is best described as

  • (a)denaturation of milk protein by acid
  • (b)hydrolysis of lactose into glucose and galactose
  • (c)oxidation of milk fat
  • (d)evaporation of water from milk

  Answer(a) denaturation of milk protein by acid — lactic acid produced by bacteria unfolds the protein, which then sets.