USA Biology Olympiad (USABO) Practice Exam

What role do sterol molecules play in cellular membranes?

1. They facilitate energy transfer.

2. They contribute to membrane fluidity.

3. They act as signaling molecules.

4. They provide structural support.

The correct answer is: They contribute to membrane fluidity.

Sterol molecules, such as cholesterol in animal cells, play a critical role in contributing to membrane fluidity. In cellular membranes, sterols are interspersed within the phospholipid bilayer. Their hydrophobic nature allows them to fit within this bilayer, where they help to maintain an optimal fluidity in the membrane structure. By fitting between the phospholipids, sterols reduce the motion of fatty acid chains at higher temperatures, thus preventing the membrane from becoming too fluid. Conversely, at lower temperatures, sterols can prevent the fatty acid chains from packing too closely together, which helps to maintain some degree of fluidity and flexibility. This unique property is crucial for various membrane functions, including the movement of molecules in and out of cells, membrane protein functionality, and overall cellular integrity. While some sterols can act as signaling molecules or provide some structural support, their primary recognized role in the context of cellular membranes is related to the regulation of fluidity. This is why the contribution to membrane fluidity is the most relevant aspect when discussing the function of sterols.