Cholesterol molecules are always identical in membranes

Steps:

• Recall fluid mosaic model: bilayer of phospholipids with embedded proteins, cholesterol, and carbohydrates.
• Phospholipids (likely structure 1) vary with different head groups and fatty acid chains.
• Proteins (likely structure 2) and carbohydrates/glycoproteins (likely structure 3) differ in sequence, function, and attachment.
• Cholesterol (structure 4) is a single uniform steroid molecule, identical across all instances.

Why D is correct:

• Cholesterol is defined as a specific, invariant lipid molecule (C27H46O) that maintains consistent structure in all membranes.

Why the others are wrong:

• A: Includes diverse phospholipids, proteins, and carbohydrates, which vary.
• B: Structure 1 (phospholipids) has multiple subtypes, not identical.
• C: Structures 2 and 3 (proteins, carbohydrates) exhibit structural diversity.

Final answer: D