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.

Question: What are its important functions?
Self-contained rewrite: What are the important functions of a cell membrane?