Answer:
a police officer who allows select people to enter and leave a crime scene.
Step-by-step explanation:
The plasma membrane has a property called selective permeability.
Selective permeability is a property of the plasma membrane that controls the entry and exit of substances in the cell, so we can compare the function of the plasma membrane with the work of a police officer who allows selected people to move in and out of a place.
Through selective permeability, the plasma membrane selects the substances that must enter and leave the cell. We can say that the membrane acts as a filter, allowing the passage of small substances and preventing or hindering the passage of large substances.