CDS (I) 2024 General Knowledge · Question 27 of 120

Correct — C, lysozyme. Lysozyme, also called muramidase, is the antibacterial enzyme of the body's watery secretions — it 'is abundant in secretions including tears, saliva, human milk, and mucus'. It works by hydrolysing the peptidoglycan mesh that gives a bacterial cell wall its strength, and that hydrolysis 'compromises the integrity of bacterial cell walls causing lysis of the bacteria'. Alexander Fleming found it in nasal secretions and named it in 1922 — six years before penicillin — recording that 'as this substance has properties akin to those of ferments I have called it a Lysozyme'. In immunity terms it belongs to the innate, non-specific defences that NCERT groups as physiological barriers, alongside the acid of the stomach.

• (a)ribozyme — A ribozyme is a catalytic RNA molecule, not a protein, and its role is in reactions such as RNA splicing — it has nothing to do with killing bacteria in saliva or tears.
• (b)lipase — Lipase is a fat-digesting enzyme. A lingual lipase does exist in saliva, but it breaks down triglycerides as part of digestion and is not antibacterial.
• (d)isomerase — Isomerase names a whole class of enzymes that rearrange a molecule into its isomer. It is a category, not a particular secretion, and none of its members is the antibacterial agent of tears.

Innate immunity is the defence you are born with, and it works in layers. Physical barriers come first — the skin, and the mucus lining the respiratory, gastrointestinal and urogenital tracts. Behind them are physiological barriers: NCERT lists the acid in the stomach, the saliva in the mouth and the tears from the eyes as agents that prevent microbial growth. Lysozyme is the enzyme that does much of that work in saliva, tears, milk and mucus, cutting the peptidoglycan of bacterial cell walls so the cell bursts.

The option list is built around word shape rather than function, so read the endings carefully. Three of the four are recognisable enzyme names, and two of them — ribozyme and lysozyme — differ by three letters, which is the whole trap. Anchor on the root: 'lyso' points to lysis, the bursting of a cell, and that is exactly what the enzyme does to a bacterium. NCERT is worth a note here: it names saliva and tears as physiological barriers but does not itself name lysozyme, so this is one of the places where a CDS science item reaches just past the school text. The enzyme's history is a useful hook — Fleming's 1922 discovery of lysozyme is the less famous half of a career whose other half was the 1928 discovery of penicillin.

• Lysozyme, also called muramidase, is abundant in tears, saliva, human milk and mucus.
• It hydrolyses peptidoglycan in the bacterial cell wall, compromising the wall's integrity and lysing the bacterium.
• Alexander Fleming discovered it in nasal secretions and named it in 1922, six years before penicillin.
• NCERT classes saliva and tears among the physiological barriers of innate immunity, along with stomach acid.

• Misreading lysozyme as ribozyme; one is a bacteria-killing protein, the other a catalytic RNA.
• Assuming every enzyme in saliva is digestive — amylase and lipase digest, lysozyme defends.
• Treating lysozyme as part of acquired immunity; it is a non-specific, innate defence and needs no prior exposure.

Asked as a one-line recall item naming the secretion and asking for the enzyme, with a near-homophone planted among the options.

• practice — not a real PYQ

  Lysozyme kills bacteria mainly by acting on which part of the bacterial cell?

  • (a)The plasmid DNA
  • (b)The peptidoglycan cell wall
  • (c)The ribosomes
  • (d)The flagellum

  Answer(b) The peptidoglycan cell wall — hydrolysing it destroys the wall's integrity and the cell lyses.

• practice — not a real PYQ

  Which one of the following is classified as a physiological barrier of innate immunity?

  • (a)Memory T cells
  • (b)Acid in the stomach
  • (c)Antibodies from a vaccine
  • (d)The skin

  Answer(b) Acid in the stomach — a physiological barrier; the skin is a physical barrier and the other two belong to acquired immunity.