Membrane fluidity needs to be maintained for the cell to function normally, example, when red blood cells try to squeeze through blood capillaries. Also, fluidity allows small molecules to diffuse rapidly through and aids cell movement, growth, division etc.
Apart from temperature, one important factor for fluidity is the length of the hydrocarbon tail of the phospholipid in the bilayer which determines the stability and fluidity of the biomembrane. The longer the chain length of the tail, the more likely the hydrocarbon tails will interact with one another forming a secure and rigid structure.
Another factor is the degree of unsaturation of the hydrocarbon tail of the phospholipid in the bilayer. The chain that has a double bond does not have the maximum number of hydrogen atoms and the double bond creates a small kink in the hydrocarbon tail (chapter 2, lipids) which makes it more difficult for the tail to pack against one another and therefore more space for movement.
Additionally, certain types of movement within the membrane are more frequent than others;
- Lateral Diffusion; molecules of the biomembrane simply transpose with neighboring molecules.
- Rotation; is when an individual lipid molecule rotates very quickly around its axis (up to 30’000 rotations per minute).
- Swing; from side-to-side.
- Flexion; contraction movement.
- Transverse Diffusion; also known as flip flop, is a movement of molecules from one half of a mono-layer to the other. The reason why flip flop happens less often is because the hydrophilic head of the lipid must go cross the internal hydrophobic sheet to go to the other mono-layer and faces a lot of friction. Flip flop is usually facilitated with the help of enzymes called flippases.
Because of flippases, different types of phospholipid molecules (Examples: Sphingomyelin, phosphatidylcholine, phosphatidylserine, phosphatidylethanol-amine, phosphatidylinositol and cholesterol) become concentrated in each monolayer making the bilayer asymmetrical. The two halves of the bilayer contain very different types of phospholipids and glucolipids, and the proteins of the bilayer have different orientations.
All these add up to create a highly diverse and fluid biomembrane.