The cell or plasma membrane is a lipid bilayer containing proteins, cholesterol, and oligosaccharides that functions as a selective barrier for entry and exit of substances. The plasma membrane, by limiting the transport of some things and facilitating the movement of others helps to maintain the internal environment of the cell, which is different from the extracellular fluid. Under an electron microscope membranes appear to have a trilaminar structure. This is because the lipid bilayers are arranged such that hydrophilic phospholipid groups are oriented toward the outside of the membrane while the more hydrophobic lipid fatty acid chains form the middle of the trilaminar structure. The molecular make up of each half of the membrane is different in that different lipids and proteins are more abundant in one side over the other.
Proteins are a very important part of the cell membrane.
Basically they can be classified into two groups based on physical distribution. Integral proteins are embedded within the cell membrane and may in fact pass multiple times through the membrane. Peripheral proteins are loosely associated with membrane surfaces. Carbohydrate portions of glycoproteins and glycolipids are found on the external surface of the cell membrane where they are important parts of receptor molecules. Receptors are necessary to cellular signalling, adhesion, and recognition.