5.13: Membrane Fluidity

Cell membranes are composed of phospholipids, proteins, and carbohydrates loosely attached to one another through chemical interactions. Molecules are generally able to move about in the plane of the membrane, giving the membrane its flexible nature called fluidity. Two other features of the membrane contribute to membrane fluidity: the chemical structure of the phospholipids and the presence of cholesterol in the membrane.

Fatty acids tails of phospholipids can be either saturated or unsaturated. Saturated fatty acids have single bonds between the hydrocarbon backbone and are saturated with the maximum number of hydrogens. These saturated tails are straight and can, therefore, pack together tightly. In contrast, unsaturated fatty acid tails contain double bonds between carbon atoms, giving them a kinked shape and preventing tight packing. Increasing the relative proportion of phospholipids with unsaturated tails results in a more fluid membrane. Organisms like bacteria and yeasts that experience environmental temperature fluctuations are able to adjust the fatty acid content of their membranes to maintain a relatively constant fluidity.

In cell membranes, cholesterol is able to interact with heads of phospholipids, partly immobilizing the proximal part of the hydrocarbon chain. This interaction decreases the ability of polar molecules to cross the membrane. Cholesterol also prevents the phospholipids from packing together tightly, thereby preventing the likelihood of membrane freezing. Likewise, cholesterol acts as a structural buffer when temperatures get to warm, limiting excessive fluidity.

Cholesterol is also proposed to have a role in the organization of membrane lipids and proteins into functional groups called lipid rafts. These groups of proteins, phospholipids, and cholesterol are thought to compartmentalize regions of the membrane, positioning molecules with similar roles in close proximity to one another. However, the specific structure and function of these membrane patches are unclear and an active area of research.

The diversity of components associated with the plasma membrane, along with its ability to adapt to change, help to maintain the membrane's dynamic fluidity. For example, one of the major components, phospholipids, can exist in either saturated forms, containing the maximum number of hydrogens and no double bonds, or unsaturated forms, which have at least one double bond. When the temperature drops, saturated phospholipids, with their long, straight, fatty acid chains, can pull closer together than the unsaturated ones, which have a kink in their chains due to the double bonds.

This extra space preserves some of the membrane fluidity. Another component, cholesterol, can insert itself between phospholipids, also creating a space that increases fluidity during colder temperatures. Under warmer temperatures, when the membrane is more fluid, the steroid rings in cholesterol provide structural support for the phospholipids, preventing the membrane from becoming too fluid. Thus, regulating membrane fluidity is an important cellular response to changes in temperature, such as when seasonal changes induce modifications in the fatty acid composition of fish.