Question

How do we know that membrane phospholipids regularly intermingle and move laterally, but rarely flip from one side of the phospholipid bilayer to the other?

Answer 1

Answer: One of the features of the fluid mosaic model is the nature of its membrane to allow its movement freely. There are mainly 3 types of movements possible for the membrane to allow, although their frequency can be different, let’s understand one by one:

1. The rotational movement which allows the interaction with its neighbor via rotation.
2. Lateral where the phosphates move along with its leaflet.
3. Flip-Flop movement of a molecule from one side to another.

(Which allows phospholipids to interact between leaflets but not to proteins)

The reason as to why phospholipids move laterally:

Lateral movement provides the membrane its fluidity .there are two important properties    which allows the lateral diffusion to occur in a more frequent basis:

Phospholipid layers are not covalently bonded they are bonded with Vander Waal interaction or by hydrogen bonding that stabilizes the lipid bilayer structure. The fatty acids chain is generally unsaturated which helps proteins and phospholipids to diffuse laterally. Secondly, the large cross-sectional area and the high degree of randomness around also contributes to lateral movement across membranes.

The reason as the membrane does not allow flip flop movements so often:

Movement of a molecule from one side to another is called flip flop and also called transverse movement which needs to overcome high energy constraint or barrier. The polar region in the case of protein is so large that it cannot make its way to the hydrophobic region of the membrane. This is why proteins cannot flip flop at all. Whereas, the phospholipids have the chance of flip flopping because of its less polar area region and that too is mediated by special proteins called Flippases which catalyze and make its movement across the membrane easy.