What is one function of cholesterol in the plasma membrane?

Cholesterol plays a crucial role in maintaining the structural integrity and functionality of the plasma membrane in cells. One of its primary functions is to enhance membrane fluidity and stability. By inserting itself into the lipid bilayer, cholesterol affects the packing of phospholipids, preventing them from becoming too rigid in low temperatures, thus maintaining flexibility. At higher temperatures, cholesterol helps to stabilize the membrane by restraining excessive movement of phospholipids, which can make the membrane too fluid.

This dual action helps the cell membrane to remain functional across a range of temperatures, ensuring that cellular processes that depend on membrane fluidity—such as the movement of proteins and lipids, as well as the activity of membrane-embedded enzymes and receptors—function optimally. The presence of cholesterol is especially important in eukaryotic cells, where it contributes to the overall dynamic nature of the membrane, balancing the necessity for both rigidity and flexibility.

The other options do not accurately reflect the role cholesterol plays in the plasma membrane. For example, it does not serve as a primary energy source or initiate immune responses, nor does it completely solidify the membrane, which could inhibit essential cellular functions.