Final answer:
A cell dies in the presence of penicillin because it undergoes lysis. Penicillin targets bacterial cell walls during their synthesis, causing the cell to burst due to osmotic pressure changes as it cannot prevent water from entering the cell.
Step-by-step explanation:
In the presence of penicillin, a cell dies because it undergoes lysis. Penicillin disrupts the synthesis of the bacterial cell wall by inhibiting the enzyme-transpeptidase, which is responsible for the last step in cell-wall biosynthesis. Without the structural support of the cell wall, bacteria are unable to prevent the influx of water, leading to osmotic imbalance. This causes the cells to swell and eventually burst.
This process is known as lysis. In contrast, cells without a rigid cell wall, like red blood cells, would lyse in a hypotonic environment but in a hypertonic environment, they would lose water and shrink, a process known as crenation. The same hypertonic environment causes plasmolysis in bacteria with a cell wall. During bacterial replication, particularly the log phase, penicillin is most effective because bacteria are actively synthesizing their cell walls during growth.