The Fluid-Mosaic Model of Membrane Structure
We have seen that biological membranes have both lipid and protein components. How do these two parts combine to produce a biological membrane? Currently, the fluid-mosaic model is the most widely accepted description of biological membranes. The term mosaic implies that the two components exist side by side without forming some other substance of intermediate nature. The basic structure of biological membranes is that of the lipid bilayer, with the proteins embedded in the bilayer structure (Figure 8.18).
These proteins tend to have a specific orientation in the membrane. The term fluid mosaic implies that the same sort of lateral motion that we have already seen in lipid bilayers also occurs in membranes. The proteins “float” in the lipid bilayer and can move along the plane of the membrane.
Electron micrographs can be made of membranes that have been frozen and then fractured along the interface between the two layers. The outer layer is removed, exposing the interior of the membrane. The interior has a granu-lar appearance because of the presence of the integral membrane proteins (Figures 8.19 and 8.20).
The fluid-mosaic model is the most usual description of membrane structure. In this model, the proteins “float” in the lipid bilayer without extensive interaction between the two.