Here's why:

* Selective Permeability: The cell membrane is selectively permeable, meaning it allows some substances to pass through while blocking others. This is crucial for maintaining the cell's internal environment.

* Phospholipid Bilayer: The cell membrane is composed of a phospholipid bilayer, which acts as a barrier between the cell's interior and the external environment. The hydrophobic tails of the phospholipids face inwards, creating a barrier that prevents polar molecules from passing through easily.

* Proteins: Embedded within the phospholipid bilayer are various proteins that play a role in transport. Some proteins act as channels, allowing specific molecules to pass through. Others act as carriers, binding to molecules and transporting them across the membrane.

Overall, the cell membrane is a dynamic structure that plays a vital role in regulating the movement of molecules in and out of the cell, ensuring its proper functioning and survival.