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.
