Question

Consider these two hypothetical bilayer systems:

1. Bilayer System A: composed of phospholipids with saturated acyl chains 20 carbons in length

2. Bilayer System B: composed of phospholipids with acyl chains of the same length as in System A but with cis double bonds as C-5, C-8, C-11, and C-14

What is the anticipated difference in T_m for Bilayer Systems A and B?

Answer 1

Ans. Higher temperature provides higher kinetic energy to the constituent phospholipid molecules in the lipid bilayer. To prevent the membrane from disintegrating apart simultaneously with preserving the optimum fluidity, the cell has relative abundance of saturated and trans-configuration (in linear shape) in the phospholipids. It facilitates tight packing of the hydrophobic tails the hydrophobic core, the membrane and its fluidity is retained.

As temperature is lowered, the kinetic energy of fatty acid chains decrease. As a result, the membrane described above becomes more rigid or less fluid at lower temperature due to greater attraction among linear fatty acid tails. To restore membrane fluidity at optimum level, the number of unsaturated as well as cis-configuration fatty acid in the phospholipids is increased. Relatively larger abundance of cis-configuration and unsaturated fatty acid in the phospholipids as well as steroids makes the membrane more fluid because both cis-form and unsaturation makes the chain irregular-shaped and hinders the closed packing of hydrophobic core.

# Result: Due to relatively higher thermal stability because of tight packing of the saturated phospholipids in the lipid bilayer, the lipid bilayer system A would have a higher melting temperature.

# Due to relatively lower thermal stability because of loose packing of the unsaturated phospholipids in the lipid bilayer, the lipid bilayer system B would have a lower melting temperature.

# In summary, lipid bilayer system A has higher melting temperature than that of bilayer system B.