The question asks about the specific component that penicillin targets in bacteria, which is absent in human cells, explaining its selective toxicity.

Penicillin's Selective Action Mechanism

Penicillin works by interfering with the synthesis of the bacterial cell wall. This rigid outer layer is essential for the survival of many bacteria, protecting them from osmotic lysis and maintaining their shape.

Why Human Cells Are Unaffected

• Human cells do not possess a cell wall. They are eukaryotic cells surrounded only by a plasma membrane.
• Since penicillin's target (the cell wall) is unique to bacteria, it does not affect human cells, making it a safe and effective antibiotic.

Analysis of Options

• Cell wall: Bacteria have a peptidoglycan cell wall that penicillin inhibits. Human cells lack this structure. This is the correct target.
• Plasma membrane: Both bacteria and human cells have a plasma membrane. Penicillin does not primarily target this structure.
• Ribosome: Bacteria have 70S ribosomes, while human cells have 80S ribosomes. Some antibiotics target ribosomes, but this is not penicillin's primary mechanism.
• Chromosome: Both cell types contain chromosomes (genetic material), but penicillin does not directly target them.

Therefore, penicillin targets the bacterial cell wall.