Here's why:

* Cell membranes are essential for all living cells. They act as a barrier, controlling what enters and exits the cell, and maintaining the cell's internal environment.

* Plant cells have an additional outer layer called the cell wall, but the cell membrane still exists beneath it.

* Animal cells do not have a cell wall, so the cell membrane is their outermost boundary.

So, while plant cells have a more complex structure, both plant and animal cells rely on the cell membrane for basic cellular functions.