Q.27 Penicillin, the first found natural antibiotie acts by
1. inhibiting protein synthesis
2. inhibiting cell wall synthesis
3. inhibiting DNA replication
4. disrupting the cell membrane
Penicillin Mechanism: First Natural Antibiotic Action Explained
Penicillin revolutionized medicine as the first discovered natural antibiotic, targeting bacterial cell walls specifically. This SEO-friendly article analyzes a common biology MCQ on its mechanism, revealing the correct answer with detailed option breakdowns for exam prep.
Correct Answer
The correct choice is option 2: inhibiting cell wall synthesis.
Penicillin’s β-lactam ring binds to penicillin-binding proteins (PBPs), blocking transpeptidase enzymes that cross-link peptidoglycan chains in bacterial cell walls. This weakens the wall during growth, causing osmotic lysis and bacterial death, while human cells remain unaffected due to lacking peptidoglycan.
Option Breakdown
Option 1 Analysis
Statement: Inhibiting protein synthesis.
Why Incorrect: Protein synthesis inhibitors like tetracycline or erythromycin target ribosomes (30S/50S subunits); penicillin does not interact with these.
Option 2 Analysis
Statement: Inhibiting cell wall synthesis.
Why Correct: It specifically halts peptidoglycan cross-linking in the final biosynthesis stage, leading to cell wall instability and lysis—effective mainly against Gram-positive bacteria. Discovered from Penicillium mold, this selective toxicity defines its bactericidal nature.
Option 3 Analysis
Statement: Inhibiting DNA replication.
Why Incorrect: DNA replication blockers like fluoroquinolones target gyrase/topoisomerase; penicillin’s action is structural, not genetic.
Option 4 Analysis
Statement: Disrupting the cell membrane.
Why Incorrect: Membrane disruptors like polymyxin alter lipid bilayers; penicillin focuses solely on cell wall enzymes, sparing membranes.
Exam Relevance
This concept appears frequently in NEET and microbiology exams, emphasizing penicillin’s role as a β-lactam antibiotic pioneer since 1928. Understanding peptidoglycan’s role clarifies resistance mechanisms like β-lactamase production.
