Question

Explain how cholesterol within the plasma membranes of animals affects membrane fluidity at different temperatures.

Answer 1

Cholesterol is the key regulator of membrane fluidity in animals. It is able to interact with and form specific complexes with phospholipids that are called lipid rafts that concentrate in specific regions of the membranes. Lipid rafts result in moderation of membrane fluidity which causes the membranes to be less fluid while also making them less vulnerable to phase transitions. Cholesterol is comprised of a steroid with an -OH hydroxy group conjugated at one end and a hydrocarbon chain at the other. The rings of the steroid and the hydrocarbon chain are able to insert themselves into the phospholipid bilayer of the membrane and participate in hydrophobic interactions while the polar hydroxy group interacts with the polar head groups of the surrounding phospholipids. Cholesterol as already said lied alongside the phospholipids in the membrane and tends to dampen the effects of temperature on the membrane. Thus, cholesterol functions as a buffer, preventing lower temperatures from inhibiting fluidity and preventing higher temperatures from increasing fluidity too much. Cholesterol extends in both directions the range of temperature in which the membrane is appropriately fluid and, consequently, functional. Cholesterol also serves other functions, such as organizing clusters of transmembrane proteins into lipid rafts.