Membrane Fluidity

What is fluidity?

The viscosity of the membrane (how easily lipids and proteins can move within the bilayer)

Why are membranes dynamic structures?

Due to the ability of phospholipids to move

What phospholipid movement occurs in membranes?

• Lateral diffusion: the movement of phospholipids within the same leaflet (or layer) of the bilayer. It allows for efficient redistribution of molecules within the same layer

• Flexion: the bending or flexing of the fatty acid tails of phospholipids. This movement contributes to the overall fluidity of the membrane and allows it to adapt to environmental changes

• Rotation: the spinning of phospholipids around their long axis. This type of movement also increases membrane fluidity and supports interactions between lipids and proteins within the membrane

• Flip-flop: the movement of a phospholipid from one leaflet of the bilayer to the opposite leaflet. This is a rare event because it requires the hydrophilic head of the phospholipid to pass through the hydrophobic core of the bilayer, which is energetically unfavorable. Enzymes like flippases and scramblases can assist in this process when needed, such as during membrane assembly or apoptosis

How is membrane fluidity regulated?

• cis double bonds make the phospholipid more difficult to pack together and make the membrane thinner (unsaturated hydrocarbon chain with cis double bonds)

• The insertion of cholesterol because their orientation means that the steroid ring region of the cholesterol stiffens the upper region of the fatty acid chain in the phospholipid. This immobilises the phospholipid so its less able to move laterally and less fluid. However, the high levels of cholesterol prevent phospholipids from compacting together and becoming too rigid