$$\rightleftharpoonup{xx}$$
$$\longleftharp{xx}$$,
$$\longrightharp{xx}$$,
Membrane fluidity is explained by the fluid mosaic model of the cell membrane, which describes the plasma membrane structure as a mosaic of components—including phospholipids, cholesterol, proteins, and carbohydrates—that gives the membrane a fluid character.
Mosaic nature of the membrane
The mosaic characteristic of the membrane helps the plasma membrane remain fluid. The integral proteins and lipids exist as separate but loosely-attached molecules in the membrane. The membrane is a relatively rigid structure that can burst if penetrated or if a cell takes in too much water. However, because of its mosaic nature, a very fine needle can easily penetrate the membrane without causing it to burst; the membrane will flow and self-seal when the needle is extracted.
Nature of phospholipids
Next is the nature of the phospholipids that the lipid bilayer is made up of; in saturated form, the fatty acids in phospholipid tails are saturated with bound hydrogen atoms, resulting in straight tails. In contrast, unsaturated fatty acids contain some double bonds between adjacent carbon atoms, causing a bend of approximately 30 degrees in the carbon string. If unsaturated fatty acids are compressed, the "kinks" in their tails push the adjacent phospholipid molecules away, maintaining some space between the phospholipid molecules. This "elbow room" helps maintain fluidity in the membrane at temperatures at which membranes with saturated fatty acid tails in their phospholipids would "freeze" or solidify. The relative fluidity of the membrane is crucial in a cold environment. A cold environment tends to compress membranes composed largely of saturated fatty acids, making them less fluid and more susceptible to rupturing.
Cholesterol
In animals, cholesterol is another factor that helps maintain membrane fluidity. It lies alongside the phospholipids in the membrane and tends to reduce the effects of temperature on the membrane. Thus, cholesterol functions as a buffer, preventing lower temperatures from inhibiting fluidity and preventing higher temperatures from increasing the fluidity too much. Cholesterol extends in both directions the range of temperature in which the membrane is appropriately fluid and functionally active. Cholesterol also serves other functions, such as organizing clusters of transmembrane proteins into lipid rafts. Membrane fluidity allows proteins and lipids to move and accumulate at sites to form lipid rafts and intercellular junctions, maintaining membrane permeability.
This text is adapted from Openstax Biology 2e, Section 5.1.
