The structure in a cell composed mostly of a phospholipid bilayer is the cell membrane.

Here's why:

* Phospholipid bilayer: This is the fundamental structure of cell membranes. It's a double layer of phospholipid molecules, with their hydrophilic (water-loving) heads facing the watery environment inside and outside the cell, and their hydrophobic (water-fearing) tails facing each other in the middle.

* Function: The cell membrane acts as a barrier, controlling what enters and exits the cell. It's also involved in cell signaling and communication.

Let me know if you have any other questions!