Here's why:

* Eukaryotic cells are complex cells with a nucleus and other membrane-bound organelles.

* The plasma membrane is a thin, flexible barrier that surrounds the entire cell.

* It acts as a gatekeeper, controlling what enters and exits the cell, maintaining the cell's internal environment, and allowing the cell to interact with its surroundings.

Other structures like the cell wall (in plants and some bacteria) and the capsule (in some bacteria) also contribute to separation but are not present in all eukaryotic cells.