The bilayer allows for rapid lateral movement of molecules due to which property?

• A. Strong covalent bonds between lipids
• B. Enzymatic cross-linking
• C. Rigid cholesterol crystals
• D. Flexibility and fluidity of fatty acids

Answer: (D)

Membranes behave as a two‑dimensional fluid because the lipid tails are flexible and held together by weak, noncovalent interactions. This fluid state lets phospholipids slide past one another, so molecules can move rapidly laterally within the bilayer. If lipids were bound by strong covalent bonds or formed rigid cross-links, movement would be restricted. Rigid cholesterol crystals aren’t what enable this mobility; cholesterol actually helps tune fluidity rather than crystallize. So the rapid lateral movement comes from the flexibility and fluidity of the fatty acid chains.