Membranes are vital because they separate the cell from the outside world. They also separate compartments inside the cell to protect important processes and events.
Cellular membranes have diverse functions in the different regions and organelles of a cell.
However, at the electron microscopic level, they share a common structure following routine preparative steps. Note the material projecting from the plasma membrane both inside and outside of the cell. Also, outside of the cell is in the upper half of photo. In the early 1930's-40's, Danielli and Davson studied triglyceride lipid bilayers over a water surface. They found that they arranged themselves with the polar heads facing outward.