Question

The fluidity of a lipid bilayer is temperature-dependent. With cooling, it undergoes gellification and loses its fluidity as

• A. Phase change happens and liquid crystals form
• B. Hydrocarbon tail stiffens at a lower temperature
• C. Bilayer gets thicker
• D. Bilayer gets thinner

Hint:

Membrane Phase Transition: Cooling below $T_m \implies$ Fatty acyl tails adopt rigid all-trans conformation and pack tightly (Gel phase Gellification).

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Membrane phase transition ($T_m$) shifts the lipid bilayer from a dynamic liquid-disordered fluid state to a rigid, tightly packed gel-solid state upon cooling.

Step 2: Detailed Explanation:
Biophysics of membrane lipid phase transition:
- At physiological temperatures above the transition temperature ($T_m$), the fatty acyl hydrocarbon tails exist in a fluid, flexible liquid-crystalline state with rapid lateral diffusion and continuous trans-gauche rotational isomerization.
- When cooled below $T_m$, thermal kinetic motion drops, and the hydrocarbon tails stiffen into fully extended all-trans conformations, packing tightly together via maximum van der Waals attractive forces.
- This cooperative immobilizing transition locks the bilayer into a rigid, non-fluid paracrystalline gel phase.

Step 3: Final Answer:
Hence, fluidity is lost as Hydrocarbon tail stiffens at a lower temperature, corresponding to option (B).