Explain penicillin's target
Outline how penicillin affects bacteria and why it does not affect viruses.
Section 10.2 · Antibiotics
Cambridge International AS & A Level Biology 9700, 2028–2030 syllabus, Version 1. Educational biology content only: it does not diagnose illness or replace advice from a qualified health professional.
Before you begin
- A pathogen is a disease-causing microorganism or infectious agent; transmission describes how it passes between hosts.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What bacterial structure is targeted by penicillin?
What you will learn
- Describe penicillin action on bacterial cell-wall synthesis
- Explain why this target is absent in viruses
Build the explanation
Penicillin is a beta-lactam antibiotic that interferes with the formation of peptidoglycan in bacterial cell walls. This weakens the wall as susceptible bacteria grow and divide; the cells may then lyse under osmotic pressure. Human cells do not have peptidoglycan cell walls, which is why this bacterial target is selective. Viruses are not cells and do not have bacterial cell walls or peptidoglycan; penicillin therefore has no target in a virus.
Apply it carefully
The syllabus asks for an outline, not a detailed molecular pharmacology account. Link target → weakened wall → effect on growing bacteria. Antibiotics do not treat viral infections by killing viruses. A patient should not self-select antibiotics; medical treatment decisions belong to qualified health professionals.
Pause and explain
Why does penicillin not treat a viral infection?
Worked example
Why does penicillin not act on a virus?
Show the worked solution
- Identify the bacterial structure it targets.
- Viruses lack bacterial peptidoglycan cell walls.
Answer Penicillin interferes with bacterial cell-wall synthesis, but viruses have no peptidoglycan cell wall, so the drug has no target in a virus.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Outline penicillin's action on a susceptible bacterium.
Give me a hint
Describe the target and consequence while the cell grows.
Compare my reasoning
- Penicillin interferes with peptidoglycan wall formation.
- The weakened wall can fail as the bacterium grows, causing lysis.
Penicillin interferes with peptidoglycan synthesis in bacterial cell walls. As a susceptible bacterium grows and divides, the weakened wall may fail under osmotic pressure and the cell may lyse.
Look for these in your work
- Target named
- Growth/division context included
- Consequence explained
A student says penicillin kills viruses because viruses reproduce inside cells. Correct the explanation.
Give me a hint
The site of reproduction is not the drug target.
Compare my reasoning
- Penicillin targets bacterial peptidoglycan wall formation.
- Viruses have no bacterial cell wall, even when they use a host cell to reproduce.
Penicillin interferes with peptidoglycan cell-wall synthesis in bacteria. Viruses are not bacterial cells and lack this wall, so reproducing inside a host cell does not create the penicillin target.
Look for these in your work
- Bacterial target stated
- Virus structure distinguished
- Host-cell point addressed
Explain why penicillin can selectively affect bacteria without targeting a human cell wall.
Give me a hint
Compare the structures present in the two cell types.
Compare my reasoning
- Bacteria have peptidoglycan cell walls.
- Human cells lack peptidoglycan walls, giving the drug a bacterial target.
Penicillin interferes with peptidoglycan formation in bacterial cell walls. Human cells do not have peptidoglycan cell walls, so this target is absent in human cells.
Look for these in your work
- Bacterial feature stated
- Human difference stated
- Selectivity linked to target
Common mistakes
- Claiming penicillin attacks every bacterium equally
- Saying viruses have cell walls
- Explaining the viral exception only by where viruses reproduce
What can you explain now?
What bacterial structure is targeted by penicillin?
Compare with the explanation
Peptidoglycan cell-wall synthesis
Penicillin interferes with building the bacterial cell wall.
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Make it stick
What does penicillin target in bacteria, and why is that target absent from viruses and human cells?
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