Scientists identified the plasma membrane in the 1890s and its principal chemical components (lipids and proteins) by 1915. The model for plasma membrane structure, proposed in 1935 by Hugh Davson and James Danielli, was the first model to be widely accepted in the scientific community. The model was based on the plasma membrane's "railroad track" appearance in early electron micrographs. Davson and Danielli theorized that the plasma membrane's structure resembled a sandwich with the analogy of proteins to bread and lipids to its filling. In the 1950s, advances in microscopy, notably transmission electron microscopy (TEM), allowed researchers to see that the plasma membrane's core consisted of a double rather than a single layer. In 1972, S.J. Singer and Garth L. Nicolson proposed a new model, "the fluid mosaic model," that describes the structure and function of the plasma membrane based on microscopic observations.
The fluid mosaic model remains the best account for plasma membrane structure and function as we now understand them. The fluid mosaic model describes the plasma membrane structure as a mosaic of components— phospholipids, cholesterols, proteins, and carbohydrates—that gives the membrane a fluid character. Plasma membranes range from 5 to 10 nm in thickness. For comparison, human red blood cells, visible via light microscopy, are approximately 8 µm wide, or about 1,000 times wider than a plasma membrane.
The model describes the plasma membrane structure as a mosaic of components in which the components can flow and change position while maintaining their fundamental integrity. The phospholipid molecules and the embedded proteins can diffuse rapidly both across and laterally in the membrane. The fluidity of the plasma membrane is also essential for the activities of certain enzymes and the transport of molecules within the membrane.
The main fabric of the membrane is composed of amphiphilic phospholipid molecules, having hydrophilic (water-loving) heads to interact with the environment and hydrophobic (water-repelling) tails within the lipid bilayer. Proteins are the second major component of plasma membranes. The proteins tend to orient alongside the phospholipids, with the hydrophobic region of the protein adjacent to the tails of the phospholipids and the hydrophilic region or regions of the protein protruding from the membrane. The proteins are in contact with the cytosol or extracellular fluid. Carbohydrates, the third major component of plasma membranes, are always found on the exterior cell surface and are bound either to proteins to form glycoproteins or to lipids to form glycolipids. These carbohydrate chains may consist of multiple monosaccharide units and can be straight or branched. In addition to peripheral proteins, carbohydrates include specialized sites on the cell surface that allow cells to recognize each other.
This text is adapted from Openstax, Concepts of Biology, Section 3.4 Fluid Mosaic Model and Openstax, Biology 2e, Section 5.1 Fluid Mosaic Model
