Question

A lipid bi-layer membrane can be described as a “fluid mosaic”, with free lateral diffusion and almost no unassisted flipping. What are the features of lipids and lipid bi-layers that contribute to these characteristics?

Answer 1

The lipids aggregate to form bilayers. The bilayer formation is mostly influenced by the hydrophobic effect of the lipids. In aqueous solutions amphiphilic molecules with hydrophilic heads and hydrophobic tails form the micells.

The hydrophobic tails repel the contact between them and water molecules but allow the salvation of polar head groups. The amphiphilic lipids forming micells are Glycerophospholipids and Sphingolipids but not the fatty acids. Fatty acids are mostly non polar in nature. They form the fluid part of the bilayer. The aggregation of micells formed by Glycerophospholipids and Sphingolipids with two hydrocarbon tails pack molecules together to form compact bilayer.

The transfer of a lipid molecule across a bilayer is known as transverse diffusion or flip-flop. This is very slow because the flip-flop requires the hydrated polar head group of lipid to pass across the anhydrous hydrocarbon core of the bilayer. The flip-flop rates of Phospholipids have half - time of several days.

The low flip-flop rates are observed in lipids that are highly mobile in the plane of bilayer. This mechanism is known as lateral diffusion. The lipids in a membrane can diffuse 1 ?m length bacterial cell in ~ 1s. The interior lipid bilayer is in constant motion due to rotation around the C-C bonds in the hydrophobic tails and has the viscosity of light machine oil.

The viscosity of bilayer increases to polar head groups that have limited rotation and lateral mobility is more constrained by interactions of polar head groups. Thus, lateral diffusion is rapid because of the exchange of phospholipid molecules in the same bilayer than the transverse diffusion in which exchange of lipids takes place from one bilayer to another bilayer.