Penicillin is selectively toxic to bacterial cells because it specifically targets the bacterial cell wall, which human cells lack. Penicillin inhibits enzymes (penicillin-binding proteins) involved in the cross-linking of peptidoglycan, a critical structural component of the bacterial cell wall. This inhibition prevents the proper synthesis of the cell wall, weakening it and causing bacterial cells to burst due to osmotic pressure. Human cells do not have peptidoglycan or cell walls, so penicillin does not affect them, making it harmless to human cells.
Selective Toxicity Explanation
- Bacteria have a cell wall made of peptidoglycan, which provides strength and prevents osmotic lysis.
- Penicillin binds to and inactivates enzymes (DD-transpeptidase) responsible for forming cross-links in peptidoglycan.
- Without these cross-links, the bacterial cell wall weakens, leading to cell death.
- Human cells lack peptidoglycan cell walls and do not possess the target enzymes, so they are unaffected by penicillin.
Summary
Penicillin's selective toxicity arises because it blocks a biochemical process unique to bacteria—the construction of peptidoglycan cell walls—leaving human cells unharmed as they do not participate in this process.
