What does the fluid mosaic model describe about the cell membrane?

• A. The membrane is a fixed rigid structure
• B. The membrane is fluid, allowing lateral movement of lipids and proteins
• C. The membrane is composed only of lipids with no proteins
• D. The membrane is a single-layered sheet of phospholipids

Answer: (B)

Membranes are dynamic, flexible barriers that aren’t fixed in place. The fluid mosaic model describes the plasma membrane as a phospholipid bilayer in which lipids and proteins move laterally within the plane of the membrane. This fluidity—lipids and many proteins sliding around—allows the membrane to rearrange itself as needed for processes like diffusion, vesicle formation, and protein function. The mosaic part refers to the variety and arrangement of proteins and other molecules embedded in or attached to the bilayer, creating a patchwork of components that work together.

This view also fits with why the membrane can function in many roles: transport channels and carriers move with the lipids, signaling receptors and enzymes are embedded in different places, and cholesterol helps modulate the membrane’s fluidity. The other descriptions don’t match how membranes actually behave: they aren’t fixed and rigid, they aren’t composed solely of lipids, and they aren’t a single-layer sheet.