Here's why:

* Selectively Permeable: The cell membrane acts like a gatekeeper, allowing some substances to pass through while blocking others. This is called selective permeability.

* Phospholipid Bilayer: The cell membrane is made up of a double layer of phospholipids, with their hydrophobic tails facing inwards and hydrophilic heads facing outwards. This structure helps control the movement of molecules.

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

The cell membrane is crucial for maintaining the cell's internal environment, ensuring the right nutrients enter and waste products exit.