Here's why:

* Living matter: Cells are the fundamental units of life. They contain all the necessary components for life, including DNA, proteins, and other molecules.

* Barrier: The cell membrane (also called the plasma membrane) is a thin, flexible barrier that encloses the cell, separating its internal environment from the external environment. This membrane controls what enters and exits the cell, maintaining its internal balance.

Let me know if you have any other questions!