What is responsible for the fluidity of the plasma membrane?

The plasma membrane is composed of a phospholipid bilayer with embedded proteins. The lipid molecules in the plasma membrane have a hydrophilic (water-loving) head and a hydrophobic (water-fearing) tail, which makes them amphipathic in nature. The tails are composed of hydrocarbon chains, which can be saturated or unsaturated. The degree of unsaturation affects the fluidity of the membrane. The more unsaturated the lipids are, the more fluid the membrane will be. This is because the unsaturated hydrocarbon chains introduce kinks in the tails, which prevents the lipids from packing tightly together. As a result, the membrane can move and change shape. Therefore, lipids are responsible for the fluidity of the plasma membrane.