The plasma membrane is a thin membrane that surrounds and defines the boundaries of all living cells. It is only about 8 nanometers (nm) thick; in fact, it would take over 8,000 plasma membranes to equal the thickness of an average piece of paper. Since cell membranes are so fragile when observed within the living organism, scientists could only propose theoretical models for the membrane’s structure. In fact, the majority of past techniques to analyze plasma membranes did not permit direct observation of their function.

But advances in the study of the membranes have been made. The current model of the plasma membrane, frequently referred to as the fluid mosaic model, is based on studies by American cell biologist Seymour J. Singer (1924–) and American biochemist and cell biologist Garth L. Nicholson (1943–). In 1972, the scientists’ research revealed that the plasma membrane is a combination of proteins bobbing in a fluid bilayer of phospholipids with various proteins attached to or embedded in it. This model has been tested repeatedly and has been shown to accurately predict the properties of many kinds of cellular membranes; this structure has also been confirmed using a technique known as freeze-fracture electron microscopy.