A Cell Membrane and the Door of a solar-powered car performs similar functions in their respective systems!
The Cell Membrane is partially permeable and controls the movement of substances entering and leaving the cell. Likewise, the Car door acts as the invisible membrane between the surrounding and the inside of the car. It only allows certain people (the keyholder) to get into the car as there is a lock system to stop others from entering as well.
However, the car door requires an external factor to prevent others from entering: the driver to press the lock. In the plant cell, the cell membrane can perform its function by itself.