A spherical structure consisting of a double layer (bilayer) of fatty acid molecules surrounding a watery interior. Protocells can be made in the laboratory and demonstrate certain properties of living cells; primarily they are compartments in which biomolecules can become concentrated and interact to form more complex biomolecules. The bilayer mimics the plasma membrane in allowing small molecules such as sugars and nucleotides to enter, but preventing nucleic acids (DNA or RNA) from leaving. It is suggested that these properties illustrate how the earliest cells might have arisen from spontaneous assembly of fatty acids. See also origin of life; synthetic biology.