Here's why:

* Structure: Phospholipids are molecules with a hydrophilic (water-loving) head and a hydrophobic (water-fearing) tail. They self-assemble into a double layer, with the hydrophilic heads facing the watery environments inside and outside the cell, and the hydrophobic tails facing each other in the center. This structure forms the basic barrier of the cell membrane.

* Function: The phospholipid bilayer provides a selective barrier, allowing some molecules to pass through while preventing others. This is essential for maintaining the cell's internal environment.

* Fluidity: The phospholipid bilayer is not rigid but fluid, allowing movement of lipids and proteins within the membrane. This fluidity is important for cell processes like cell signaling and growth.

While phospholipids are the primary component, cell membranes also contain other important molecules like:

* Proteins: Embedded in the membrane, they perform various functions like transport, communication, and structural support.

* Cholesterol: A type of lipid that helps maintain membrane fluidity and stability.

* Carbohydrates: Attached to lipids and proteins, they play roles in cell recognition and adhesion.

So, while the phospholipid bilayer is the fundamental building block, the cell membrane is a complex and dynamic structure with multiple components working together to maintain cellular function.