<P> Previous models of biological membranes included the Robertson Unit Membrane Model and the Davidson - Danielli Tri-Layer model . These models had proteins present as sheets neighboring a lipid layer, rather than incorporated into the phospholipid bilayer . Other models described repeating, regular units of protein and lipid . These models were not well supported by microscopy and thermodynamic data, and did not accommodate evidence for dynamic membrane properties . </P> <P> An important experiment that provided evidence supporting fluid and dynamic biological was performed by Frye and Edidin . They used Sendai virus to force human and mouse cells to fuse and form a heterokaryon . Using antibody staining, they were able to show that the mouse and human proteins remained segregated to separate halves of the heterokaryon a short time after cell fusion . However, the proteins eventually diffused and over time the border between the two halves was lost . Lowering the temperature slowed the rate of this diffusion by causing the membrane phospholipids to transition from a fluid to a gel phase . Singer and Nicholson rationalized the results of these experiments using their fluid mosaic model . </P> <P> The fluid mosaic model explains changes in structure and behavior of cell membranes under different temperatures, as well as the association of membrane proteins with the membranes . While Singer and Nicolson had substantial evidence drawn from multiple subfields to support their model, recent advances in fluorescence microscopy and structural biology have validated the fluid mosaic nature of cell membranes . </P> <P> Additionally, the two leaflets of biological membranes are asymmetric and divided into subdomains composed of specific proteins or lipids, allowing spatial segregation of biological processes associated with membranes . Cholesterol and cholesterol - interacting proteins can concentrate into lipid rafts and constrain cell signaling processes to only these rafts . Another form of asymmetry was shown by the work of Mouritsen and Bloom in 1984, where they proposed a Mattress Model of lipid - protein interactions to address the biophysical evidence that the membrane can range in thickness and hydrophobicity of proteins . Phospholipids can be asymmetric through the active translocation of a phospholipid type from one leaflet of the membrane to the other . This process is controlled by flippase and scramblase enzymes . A biological example of this is the segregation of phosphatidylserine to be highly enriched on the cytosolic side of the plasma membrane . Phosphatidylserine localization to the outer leaflet can trigger immune responses . </P>

Describe the fluid mosaic model of the molecular structure of the plasmalemma