* Plasma membrane: This is a thin, flexible barrier that surrounds the cell and regulates what enters and exits. It's essential for maintaining the cell's internal environment and allowing necessary substances to pass in and out.

Here's a breakdown:

* Plant cells: Plant cells have a plasma membrane, but they also have a rigid cell wall made of cellulose that provides additional support and structure.

* Animal cells: Animal cells have only a plasma membrane for their outer boundary.

* Bacterial cells: Bacteria also have a plasma membrane, and many have an additional cell wall made of peptidoglycan, which differs in structure from the plant cell wall.

So, even though there are differences in their cell walls, all three types of cells have a plasma membrane as the essential barrier that separates the cell's internal environment from the outside world.