Explain how the structure of the cell wall in prokaryotes differs from that in plant cells, and outline why this difference is clinically significant.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Prokaryote cell walls are made of peptidoglycan (murein), a polymer of sugars cross‑linked by short peptide chains. Plant cell walls are composed of cellulose, a polysaccharide of β‑1,4‑linked glucose monomers, with no peptide components.
Antibiotics such as penicillin inhibit the enzymes that cross‑link the peptide chains in peptidoglycan, disrupting cell wall synthesis in bacteria.
Because human cells have no cell wall and plant cells lack peptidoglycan, these antibiotics selectively kill bacteria without harming host cells, giving selective toxicity.
Antibiotics such as penicillin inhibit the enzymes that cross‑link the peptide chains in peptidoglycan, disrupting cell wall synthesis in bacteria.
Because human cells have no cell wall and plant cells lack peptidoglycan, these antibiotics selectively kill bacteria without harming host cells, giving selective toxicity.
Examiner tips
- Use the exact terms ‘peptidoglycan’ and ‘cellulose’. Explain the role of peptide cross‑links and the target of penicillin. Show why the absence of peptidoglycan in host cells gives selective toxicity. Keep the answer concise and to the point.
Common mistakes
- Confusing peptidoglycan with cellulose. Failing to mention the peptide cross‑links. Losing the point about selective toxicity by not stating that human cells lack a cell wall.
Mark scheme (4 marks)
- Prokaryote cell walls are composed of peptidoglycan (murein), a polymer of sugars cross-linked by short peptide chains.
- Plant cell walls are composed of cellulose, a polysaccharide of β-1,4-linked glucose monomers, with no peptide components.
- Antibiotics such as penicillin target peptidoglycan synthesis / inhibit enzymes that cross-link the peptide chains, disrupting cell wall formation in bacteria.
- Because human (animal) cells lack a cell wall entirely, and plant cells lack peptidoglycan, these antibiotics selectively kill bacteria without harming host cells, providing selective toxicity.
Key terms in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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