These myriad abilities are due to the membrane's composition. In 1972, S. Jonathan Singer and Garth Nicolson of the University of California, San Diego proposed a model to describe the relationship of proteins and lipids in an idealized membrane. Although such tripartite proteins were unknown at the time Singer and Nicolson proposed their model, they have since been shown to play important roles in a number of biological processes, including those that involve the transport of molecules into the cell. About half of the molecules in an average membrane are phospholipids. In a surface membrane, phospholipids spontaneously arrange themselves into a double layer with the phosphate heads touching the watery interior and exterior of the cell, and the lipid tails buried in the middle of the layer.
Cholesterol is a rigid molecule that helps stabilize the membranes of animal cells.