A-Level Biology May/June 2024 Q3(b): Compare lysozyme and penicillin to show the similarities and differences between these…

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  Similarities

  Both lysozyme and penicillin are antibacterial agents that target the peptidoglycan cell wall of bacteria. They weaken the wall, which means it can no longer withstand the cell's internal turgor pressure. This leads to the cell taking in excess water by osmosis, swelling, and ultimately bursting in a process called lysis.

  Differences

  1. Lysozyme is a protein that functions as an enzyme, catalysing the breakdown of the cell wall. In contrast, penicillin is an antibiotic that acts as a non-competitive enzyme inhibitor.

  2. Their specific modes of action are different. Penicillin inhibits the enzyme transpeptidase, preventing it from forming peptide cross-bridges between peptidoglycan chains as the cell wall is being synthesised. Lysozyme, however, acts on the existing wall structure by directly hydrolysing the glycosidic bonds within the peptidoglycan molecules themselves.

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  How the marks are awarded

  • B1 — Stating the similarity that both agents weaken the peptidoglycan cell wall, causing the cell to burst/lyse due to its inability to withstand turgor pressure.
  • B1 — Identifying a key difference in their chemical nature: lysozyme is an enzyme (protein), whereas penicillin is an antibiotic that functions as an enzyme inhibitor.
  • B1 — Describing the difference in their specific mechanism: penicillin inhibits transpeptidase to prevent peptide cross-bridge formation, while lysozyme hydrolyses glycosidic bonds in the peptidoglycan.

  Common mistakes

  • Stating that penicillin breaks existing cross-bridges; it only prevents the formation of new ones in growing bacteria.
  • Confusing the roles, for example, stating that lysozyme inhibits an enzyme or that penicillin hydrolyses bonds.
  • Providing vague answers like 'they both kill bacteria' without mentioning the cell wall, peptidoglycan, or lysis, which is insufficient for a mark.
  • Incorrectly classifying lysozyme as an antibiotic; it is a naturally produced defensive enzyme.

  Examiner tip: For comparison questions, always address both similarities and differences with specific details, using comparative language like 'whereas' or 'in contrast' to make your points clear.

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