Here's why:

* Structure: The phospholipid bilayer is a double layer of phospholipid molecules, arranged with their hydrophilic (water-loving) heads facing outward towards the watery environment inside and outside the cell, and their hydrophobic (water-fearing) tails facing inward, forming a barrier between the two environments.

* Function: This structure creates a semi-permeable barrier, allowing small, nonpolar molecules to pass through but restricting the passage of larger and polar molecules. This crucial function is essential for maintaining the cell's internal environment and regulating the exchange of substances.

While other components, like proteins and cholesterol, are also crucial for the cell membrane's functionality, the phospholipid bilayer forms the fundamental structural basis of the membrane.