Final answer:
Cholesterol helps regulate the fluidity of the plasma membrane, making it stable and less permeable to certain molecules.
Step-by-step explanation:
Plasma membranes must allow certain substances to enter and leave a cell, and prevent some harmful materials from entering and some essential materials from leaving. In other words, plasma membranes are selectively permeable—they allow some substances to pass through, but not others. If they were to lose this selectivity, the cell would no longer be able to sustain itself, and it would be destroyed.
One of the ways in which plasma membranes maintain their selective permeability is through the presence of cholesterol. Cholesterol helps regulate the fluidity of the plasma membrane, making it more stable and less permeable to certain molecules. Without cholesterol, plasma membranes would be too fluid, not firm enough, and too permeable to some molecules.