Here's a breakdown of why:

* Phospholipids are amphipathic: They have a hydrophilic (water-loving) head and a hydrophobic (water-fearing) tail. This unique structure allows them to form a double layer in water.

* The structure of the bi-layer: The hydrophilic heads of the phospholipids face outwards, interacting with the watery environments inside and outside the cell. The hydrophobic tails face inwards, forming a barrier that prevents water-soluble molecules from passing through easily.

* Fluid mosaic model: The cell membrane isn't a static structure. The phospholipids are constantly moving, creating a fluid, dynamic barrier. This fluidity allows the membrane to be flexible and responsive to changes in the environment.

While phospholipids are the primary component, other molecules contribute to the structure and function of the cell membrane:

* Cholesterol: Embedded within the phospholipid bilayer, cholesterol helps maintain the fluidity and stability of the membrane.

* Proteins: These are embedded within the membrane and perform various functions like transport, communication, and enzymatic activity.

So, to summarize, the phospholipid bi-layer is the fundamental structural unit of a cell membrane, providing the barrier that separates the inside of the cell from the outside environment.