Which statement best describes the fluid mosaic model?

• A. A phospholipid bilayer with embedded proteins that is flexible and selectively permeable.
• B. A rigid carbohydrate layer without proteins.
• C. A single layer of lipids with no movement.
• D. A fully solid membrane with no permeability.

Answer: (A)

The main idea tested is that the cell membrane is a dynamic, phospholipid bilayer with proteins embedded in it, giving a flexible barrier that is selectively permeable. The bilayer creates a hydrophobic interior that blocks most polar substances, while the hydrophilic heads face water on both sides. Proteins scattered throughout the membrane act as channels, carriers, receptors, and enzymes, forming a mosaic of components. The lipids and many proteins can move laterally, giving the membrane its fluid quality, which is essential for functions like transport and signaling. This combination explains selective permeability: small nonpolar molecules diffuse easily, whereas ions and larger polar molecules require specific transport proteins. Other descriptions miss key features—the membrane is not a rigid carbohydrate layer, not a single static lipid layer with no movement, and not a fully solid barrier with no permeability.