How does the fluid mosaic model describe the cell membrane structure?

• A. It describes a rigid carbohydrate wall.
• B. It describes a solid protein sheet with no lipids.
• C. It describes a single layer of lipids without proteins.
• D. It describes a phospholipid bilayer with embedded proteins that is flexible and selectively permeable.

Answer: (D)

The main idea tested is that the cell membrane is a dynamic, phospholipid bilayer with embedded proteins, making it flexible and selectively permeable. In this model, phospholipids organize into two layers with hydrophilic heads facing outward and hydrophobic tails tucked inside, creating a semi-permeable barrier. Proteins are interspersed throughout the bilayer, some spanning it (integral) and others attached to the surface (peripheral), providing channels, transporters, receptors, and structural support. Cholesterol sits among the lipids to help tune fluidity, so the membrane remains flexible across temperature changes. Because lipids and proteins can move laterally, the membrane is not rigid but fluid, allowing components to diffuse and reorganize. The combination of a lipid barrier with embedded proteins yields selective permeability: small nonpolar molecules pass easily through the bilayer, while polar molecules and ions require specific transport proteins. This description best captures both the structure and function of the cell membrane.