Biology · Cell Structure and Function

Cholesterol helps to regulate membrane fluidity over the range of physiological temperatures. It has a hydroxyl group that interacts with the polar head groups of the membrane phospholipids and sphingolipids. Therefore, the correct answer is D.

a) Controlling active transport:

Cholesterol does not directly control active transport. Active transport refers to the movement of molecules or ions across a cell membrane against their concentration gradient, which requires the use of energy. Cholesterol primarily plays a role in maintaining the integrity and fluidity of the cell membrane. 

b) Acting as a membrane blood receptor:

Cholesterol does not act as a membrane blood receptor. Receptors on the cell membrane are responsible for recognizing and binding specific molecules, such as hormones or signaling molecules, to initiate cellular responses. Cholesterol, however, is not involved in this specific function. 

c) Helping cells join together:

Cholesterol does not directly help cells join together. Cell adhesion molecules and specialized proteins are primarily responsible for cell-to-cell adhesion and joining tissues together. 

Cholesterol modulates the bilayer structure of biological membranes in multiple ways. It changes the fluidity, thickness, compressibility, water penetration and intrinsic curvature of lipid bilayers.Cholesterol contains a tetracyclic ring structure and adopts a trans conformation, rendering the molecule a planar and rigid characteristics that increases membrane packing and contributes to the integrity and rigidity of cell membranes.