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In animal cells, the cell membrane establishes this separation alone, whereas in yeast, bacteria and plants an additional cell wall forms the outermost boundary, providing primarily mechanical support. The plasma membrane is only about 10 nm thick and may be discerned only faintly with a transmission electron microscope. One of the key roles of the membrane is to maintain the cell potential.

1 A Fluid Mosaic

The basic composition and structure of the plasma membrane is the same as that of the membranes that surround organelles and other subcellular compartments. The foundation is a phospholipid bilayer, and the membrane as a whole is often described as a 'fluid mosaic' - a two-dimensional fluid of freely diffusing lipids, dotted or embedded with proteins which may function as channels or transporters across the membrane, or as receptors. Some of these proteins simply adhere to the membrane (extrinsic or peripheral proteins), while others might be said to reside within it or to span it (intrinsic proteins -- more at integral membrane protein). Glycoproteins have carbohydrates attached to their extracellular domains. Cells may vary the variety and the relative amounts of different lipids to maintain the fluidity of their membranes despite changes in temperature. Cholesterol molecules (in case of eukaryotes) or hopanoids (in case of prokaryotes) in the bilayer assist in regulating fluidity.

2 Detailed Structure

In fact, not all lipid molecules in the cell membrane are "fluid," in the sense of free to diffuse. Lipid rafts and caveolaeIn biology, caveolae ( Latin for little caves are small invaginations of the plasma membrane in many cell types, especially in endothelial cells. Some cell types, like neurons, completely lack caveolae. These flask-shaped structures are rich in proteins a are examples of more cohesive membrane regions. Across the membrane globally, also many proteins are not entirely free to diffuse. The cytoskeletonThe cytoskeleton is a cellular " scaffolding" or " skeleton" contained, as all other organelles, within the cytoplasm. It is a dynamic structure that maintains cell shape, enables some cell motion (using structures such as flagella and cilia), and plays i undergirds the cell membrane and provides anchoring points for integral membrane proteins. Anchoring restricts them to a particular cell face or surface--for example, the "apical" surface of epithelial cells that line the vertebratePetromyzontidae ( lampreys) Placodermi extinct Chondrichthyes (cartilaginous fish) Acanthodii extinct Actinopterygii (ray-finned fish) Actinistia ( coelacanths) Dipnoi ( lungfish) Amphibia ( amphibians) Reptilia ( reptiles) Aves ( birds) Mammalia ( mammal gutFor the Physics term 'GUT', please refer to Grand unification theory The gastrointestinal or digestive tract also referred to as the GI tract or the alimentary canal or the gut is the system of organs within multicellular animals which takes in food, dige--and limits how far they may diffuse within the bilayer. Finally, rather than presenting always a formless and fluid contour, the plasma membrane surface of cells may show structure. Returning to the example of epithelial cells in the gut, the apical surfaces of many such cells are dense with involutions, all similar in size. The finger-like projections, called "microvilli", increase cell surface area and facilitate the absorption of molecules from the outside. Synapsenerve cells to communicate with one another through axons and dendrites, converting electrical signals into chemical ones. For the technology festival, see Synapse Festival. Synapses are specialized junctions through which cells of the nervous system signs are another example of highly structured membrane.