Q.29 Mammalian cells cultured at low temperature (25 to 30 C) leads to an increased
sterol content in the membrane. Elevated sterols in the membrane results in
(A) an enhancement in membrane fluidity.
(B) stabilization of membrane proteins.
(C) an increase in membrane permeability to water.
(D) a decrease in membrane fluidity.

The correct answer is (D) a decrease in membrane fluidity. Mammalian cells increase sterol (primarily cholesterol) content in their membranes when cultured at low temperatures (25-30°C) to counteract excessive rigidity from cold-induced phospholipid packing. Elevated sterols reduce membrane fluidity under these conditions, maintaining optimal membrane function.​

Option Analysis

Elevated sterols at low temperatures pack tightly with phospholipids, ordering acyl chains and restricting motion.

• (A) Enhancement in membrane fluidity: Incorrect. At low temperatures, sterols decrease fluidity by preventing phospholipid packing that would otherwise gel the membrane; they increase fluidity only relative to pure phospholipid bilayers at that cold.​

• (B) Stabilization of membrane proteins: Incorrect as primary effect. Sterols can modulate protein function via rafts or direct binding, but the direct result is biophysical ordering, not stabilization.​

• (C) Increase in membrane permeability to water: Incorrect. Sterols reduce water permeability by filling voids and raising the free energy barrier for permeation.​

• (D) Decrease in membrane fluidity: Correct. Cholesterol’s rigid rings reduce fluidity at physiological/low temperatures by increasing order parameter and viscosity.​

Mammalian cells low temperature sterol content membrane adaptation is a key concept in cell biology for CSIR NET Life Sciences. When cultured at low temperature (25 to 30°C), these cells elevate sterol (cholesterol) levels in their plasma membrane to fine-tune biophysical properties.​

Sterol Role at Low Temperature

Cholesterol acts as a fluidity buffer. At low temperature (25-30°C), it intercalates between phospholipids, promoting ordered packing that decreases membrane fluidity and prevents gel-phase transition. This maintains functionality despite cold-induced chain alignment.​

Why Decreased Fluidity Occurs

Elevated sterols fill hydrophobic voids, increase trans conformers in chains, and raise membrane viscosity. Studies show cholesterol reduces shear viscosity and diffusion rates, especially below physiological temperatures.​

• Reduces permeability to solutes like water​

• Forms liquid-ordered domains​

• Buffers against thermal extremes​

Exam Relevance

For CSIR NET, recognize cholesterol’s temperature-dependent effects: decreases fluidity at low/high temperatures relative to pure lipids, optimizing protein function and barrier properties.