An artificial membrane lacking sterols stiffens as it cools toward its phase transition. What does cholesterol normally prevent as temperatures fall?

Core Concept

As temperature drops, phospholipid tails lose motion and pack into a gel-like arrangement, destroying permeability and protein mobility. Cholesterol's bulky ring system wedges between the tails and blocks their close approach, keeping the interior disordered enough to keep working. The same molecule restrains excessive motion at high temperature, so sterols buffer fluidity in both directions rather than simply raising or lowering it.