The function of cholesterol in cell membranes is to maintain their structural integrity and fluidity. Cholesterol is a lipid molecule that resides within the phospholipid bilayer of cell membranes, helping to stabilize membrane structure, especially in varying temperatures. It prevents the membranes from becoming too fluid at high temperatures and too rigid at low temperatures. Moreover, cholesterol is vital for the formation of lipid rafts, which are microdomains that facilitate cell signaling and interactions with other cells, contributing to overall cellular functionality.