Final answer:
Penicillin halts bacterial infections by interfering with the synthesis of cell walls in reproducing bacteria.
Step-by-step explanation:
Penicillin functions by interfering with the synthesis of cell walls of reproducing bacteria. It does so by inhibiting an enzyme-transpeptidase that catalyzes the last step in bacterial cell-wall biosynthesis. The defective walls cause bacterial cells to burst. Human cells are not affected because they have cell membranes, not cell walls.