Advanced Placement (AP) Biology Practice Exam

Which role does cholesterol play in animal cells?

1. It is a source of immediate energy

2. It keeps the membrane fluid and flexible

3. It provides structural rigidity

4. It serves as a primary energy storage molecule

The correct answer is: It keeps the membrane fluid and flexible

Cholesterol plays a crucial role in maintaining the fluidity and flexibility of animal cell membranes. It is inserted into the phospholipid bilayer, where it interacts with the fatty acid tails of the phospholipids. This interaction helps to prevent the fatty acids from packing too closely together, which would make the membrane too rigid. By maintaining the right level of fluidity, cholesterol ensures that the membrane remains functional, allowing for the proper movement of proteins and lipids within the membrane and facilitating cell signaling, transport of materials, and the overall fluidity necessary for various cellular processes. Furthermore, the flexibility of the membrane is important for the function of membrane proteins and for the ability of cells to change shape, which is vital for processes such as endocytosis and cell movement. This balance is particularly important at varying temperatures, as cholesterol helps to stabilize the membrane's structure, ensuring that it remains functional in both cold and warm environments.