<P> Chemically a cell membrane is composed of four components: Phospholipids, Proteins, Carbohydrates, Cholesterol </P> <P> The fluid property of functional biological membranes had been determined through labeling experiments, x-ray diffraction, and calorimetry . These studies showed that integral membrane proteins diffuse at rates affected by the viscosity of the lipid bilayer in which they were embedded, and demonstrated that the molecules within the cell membrane are dynamic rather than static . </P> <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>

Diagram of fluid mosaic model of plasma membrane