Here's why:

* Selective Permeability: The cell membrane is selectively permeable, meaning it controls what substances can enter and exit the cell. This is crucial for the cell's survival and proper functioning.

* Phospholipid Bilayer: The cell membrane is composed of a phospholipid bilayer, which acts as a barrier. The phospholipids have a hydrophilic (water-loving) head and a hydrophobic (water-fearing) tail. This structure allows small, non-polar molecules like oxygen and carbon dioxide to pass through easily, but blocks larger, polar molecules like sugars and ions.

* Proteins: Embedded within the membrane are various proteins that play a key role in transport. Some proteins form channels that allow specific ions to pass through, while others act as carriers, actively transporting molecules across the membrane.

In summary: The cell membrane is the gatekeeper of the cell, allowing certain molecules to pass through while blocking others, ensuring the cell maintains its internal environment and functions properly.