5.3: The Fluid Mosaic Model
The fluid mosaic model was first proposed as a visual representation of research observations. The model comprises the composition and dynamics of membranes and serves as a foundation for future membrane-related studies. The model depicts the structure of the plasma membrane with a variety of components, which include phospholipids, proteins, and carbohydrates. These integral molecules are loosely bound, defining the cell’s border and providing fluidity for optimal function.
The most abundant component of the fluid mosaic model is lipids. Lipids include both phospholipids and cholesterols. Phospholipids are amphipathic, having both hydrophobic and hydrophilic parts. They consist of a hydrophilic—water-loving—head, and two hydrophobic—water-fearing—fatty acid tails. Phospholipids spontaneously form a lipid bilayer that separates the inside of the cell from the outside. The lipid bilayer consists of the hydrophobic tails facing inward and the hydrophilic heads facing the aqueous environment inside and outside the cell. Cholesterols are a class of steroids that play a role in regulating membrane fluidity and flexibility. Membrane fluidity facilitates the transport of specific molecules and ions across the plasma membrane.
The second major component of the mosaic is proteins. Proteins can differentially associate with the lipid bilayer. For instance, some are entirely integrated into the membrane, like integrins that serve as transmembrane receptors, and transport proteins that shuttle molecules across membranes. Such integrated proteins are referred to as integral proteins. Other proteins can be found only on the surface of the cell or in the cytosol, as is the case with estrogen receptors. These proteins are referred to as peripheral proteins.
The last component of the fluid mosaic model is carbohydrates. They are located on the exterior surface of the membrane where they are bound to proteins to form glycoproteins, or to phospholipids to form glycolipids. These carbohydrate complexes are referred to as the glycocalyx—the sugar coating of the cell. Some carbohydrates in the mosaic also play essential roles as markers allowing cells to distinguish between self (cells of the same organism) and non-self (intruding foreign cells or particles).
Together, these components create a cell’s plasma membrane, with a thickness ranging between five to ten nanometers. Plasma membranes interact with their surroundings to carry out many essential processes to maintain cellular function and homeostasis.