The most widely accepted model for membrane structure is the fluid mosaic model of Singer and Nicholson, which proposes that membranes are lipid bilayers within which proteins float.
The model is based on studies of eukaryotic and bacterial membranes and was established using a variety of experimental approaches, including transmission electron microscopy (TEM) and atomic force microscopy.
Cell membranes are very thin structures, about 5 to 10 nm thick, that look like two dark lines on either side of a light interior when imaged by TEM. This characteristic appearance is evidence that the membrane is composed of two sheets of molecules arranged end-to-end. Cleavage of membranes by freeze-etching, a technique that allows the microscopist to see fine detail, exposes the proteins lying within the membrane lipid bilayer.
The chemical nature of membrane lipids is critical to their ability to form bilayers. Most membrane-associated lipids (e.g., the phospholipids are shown in the figure) are amphipathic: they are structurally asymmetric, with polar and nonpolar ends.
The polar ends interact with water and are hydrophilic; the nonpolar hydrophobic ends are insoluble in water and tend to associate with one another. In aqueous environments, amphipathic lipids can interact to form a bilayer.
The outer surfaces of the bilayer are hydrophilic, whereas hydrophobic ends are buried in the interior away from the surrounding water Two types of membrane proteins have been identified based on their ability to be separated from the membrane.
Peripheral membrane proteins are loosely connected to the membrane and can be easily removed. They are soluble in aqueous solutions and make up about 20 to 30% of total membrane protein.
The remaining proteins are integral membrane proteins. These are not easily extracted from membranes and are insoluble in aqueous solutions when freed of lipids. Integral membrane proteins, like membrane lipids, are amphipathic; their hydrophobic regions are buried in the lipid while the hydrophilic portions project from the membrane surface.
Integral membrane proteins carry out some of the most important functions of the membrane. Many are transport proteins used to move materials either into or out of the cell. Others are involved in energy-conserving processes, such as the proteins found in electron transport chains. Those integral membrane proteins with regions exposed to the outside of the cell enable the cell to interact with its environment.
Although most aspects of the fluid mosaic model are well supported by experimentation, some are being questioned. The fluid mosaic model suggests that membrane lipids are homogeneously distributed and that integral membrane proteins are free to move laterally within the membrane.
However, the presence of microdomains enriched for certain lipids and the observation that some integral proteins are present at only certain sites do not support this view. Although the term mosaic initially referred to the clusters of proteins embedded in a homogeneous lipid bilayer, it may be more accurate to use the term to refer to the patchwork of lipid microdomains found in membranes. Research is ongoing to determine the physiological role of these microdomains.