Here's why:

* Structure: Phospholipids are the building blocks of the cell membrane. They have a unique structure with a hydrophilic (water-loving) head and hydrophobic (water-fearing) tails. This allows them to form a bilayer where the heads face the watery environments inside and outside the cell, while the tails form a hydrophobic barrier in the middle.

* Fluid Mosaic Model: The cell membrane isn't a rigid structure, but rather a fluid mosaic. Phospholipids can move laterally within the membrane, creating a dynamic and flexible structure.

* Function: The phospholipid bilayer serves as a barrier, controlling what enters and exits the cell. It also provides a platform for other membrane components like proteins and cholesterol.

While phospholipids are the most abundant component, the cell membrane also contains other important molecules, including:

* Proteins: They are responsible for various functions like transport, signaling, and adhesion.

* Cholesterol: It helps to regulate membrane fluidity and stability.

* Carbohydrates: They are often attached to lipids or proteins and play roles in cell recognition and signaling.