Here's why:

* The cell membrane is a barrier: It surrounds the entire cell, separating the inside (cytoplasm) from the outside environment.

* Selective permeability: The cell membrane is selectively permeable, meaning it controls which substances can enter or leave the cell. It allows some molecules to pass through easily while blocking others.

* Transportation methods: The cell membrane uses various methods to transport materials, including:

* Passive transport: Movement of substances across the membrane without requiring energy (e.g., diffusion, osmosis).

* Active transport: Movement of substances against their concentration gradient, requiring energy (e.g., pumps, endocytosis, exocytosis).

In summary, the cell membrane is the gatekeeper of the cell, regulating the flow of materials in and out, ensuring the cell's survival and function.