The structure of cells that provides its boundaries is called the cell membrane or plasma membrane.

Here's why:

* Boundary: The cell membrane acts as a barrier, separating the internal environment of the cell from the external environment.

* Selective Permeability: It controls what enters and exits the cell, allowing some substances to pass through while blocking others. This is essential for maintaining the cell's internal environment and carrying out its functions.

While the cell wall is also a boundary structure found in plant cells, bacteria, and fungi, it's not present in animal cells. The cell membrane is the universal boundary for all cells.