Conversation so far:
Describe cell membranes.
The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates the interior of all cells from the outside environment (the extracellular space) which protects the cell from its environment.
What are the important functions of a cell membrane?
The primary function of the plasma membrane is to protect the cell from its surroundings. Composed of a phospholipid bilayer with embedded proteins, the plasma membrane is selectively permeable to ions and organic molecules and regulates the movement of substances in and out of cells.

Question: Tell me about the phospholipid bilayer.
Self-contained rewrite: Tell me about the phospholipid bilayer.