The correct answer is option 1: cell wall. Penicillin is an antibiotic that is known to inhibit the synthesis of the bacterial cell wall. Bacterial cells have a unique structure known as a cell wall, which helps to maintain their shape and provides protection against the external environment. The cell wall is composed of peptidoglycan, a complex molecule that is essential for the integrity of the cell wall.

Penicillin works by targeting and inhibiting the enzyme transpeptidase, which plays a crucial role in the formation of cross-links between the peptidoglycan chains in the cell wall. By inhibiting this enzyme, penicillin prevents the cross-linking of peptidoglycan, thereby weakening and eventually causing the cell wall to break down. As a result, the bacteria lose their structural integrity and are unable to survive.

It is important to note that while penicillin is effective against bacterial cell walls, it does not affect the cell walls of human cells or other eukaryotic organisms. This is because eukaryotic cells have a different type of cell wall or do not possess a cell wall at all.