A Levels Biology (9700)•9700/11/O/N/24

Explanation
Penicillin targets bacterial cell wall synthesis
Steps:
- Identify that penicillin kills bacteria by inhibiting peptidoglycan cross-linking in cell walls.
- Recall bacterial cells contain peptidoglycan (structure 1), penicillin-binding proteins (structure 2), and active transpeptidases (structure 4).
- Confirm eukaryotic cells lack these, so only bacterial cells are affected.
- Match to choices: A includes all relevant bacterial targets (1, 2, 4).
Why A is correct:
- Penicillin binds irreversibly to penicillin-binding proteins, preventing peptidoglycan synthesis essential for bacterial survival (definition of beta-lactam antibiotic action).
Why the others are wrong:
- B omits structure 4 (transpeptidases), incomplete for full mechanism.
- C omits structure 2 (binding proteins), missing key enzyme target.
- D includes only structure 4, ignoring peptidoglycan and proteins as primary sites.
Final answer: A
Topic: Antibiotics
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