Question

Describe the influences and forces driving the formation of the lipid bilayer. Briefly
discuss the effect of a phase transition on a lipid bilayer.

Answer 1

The lipid bilayer (or phospholipid bilayer) is a thin polar membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around all cells. The lipid bilayer is typically about five nanometers to ten nanometers thick and surrounds all cells providing the cell membrane structure.

The lipid bilayer formation is spontaneous since the hydrophobic interactions are energetically favorable to the structure. The lipid bilayer is a noncovalent assembly. The proteins and lipid molecules are held together by non covalent interactions such as Van der Waals forces (which holds the hydrophobic tails together) and hydrogen bonding (which binds the hydrophilic heads with water), which help to stabilize the lipid bilayer structure. In other words the major driving force for the formation of a lipid bilayer is hydrophobic forces on the phospholipid fatty acid carbon chains: hydrogen bonds electrostatic attractions and vander waals contacts.

One of the most important properties of a lipid bilayer is the relative mobility of the individual lipid molecules and change in mobility with temperature. This is known as the phase behavior of the bilayer. At a given temperature a lipid bilayer can exist in either a liquid or a solid phase. The solid phase is commonly referred to as a “gel” phase. All lipids have a characteristic temperature at which they undergo a transition (melt) from the gel to liquid phase. In both phases the lipid molecules are constrained to the two dimensional plane of the membrane, but in liquid phase bilayers, the molecules diffuse freely within this plane. Thus, in a liquid bilayer a given lipid will rapidly exchange locations with its neighbor millions of times a second and will, through the process of a random walk, migrate over long distances.