Self-assembled vesicles are essential components of primitive cells. The second law of thermodynamics requires that the universe becomes increasingly disordered (entropy), yet life is distinguished by its great degree of organization. Therefore, a boundary is needed to separate life processes from non-living matter. This fundamental necessity is underpinned by the universality of the cell membrane which is the only cellular structure found in all organisms on Earth.
In the aqueous environment in which all known cells function, a non-aqueous barrier is required to surround a cell and separate it from its surroundings. This non-aqueous membrane establishes a barrier to free diffusion, allowing for regulation of the internal environment within the barrier. The necessity of thermodynamically isolating a subsystem is an irreducible condition of life. In modern biology, such isolation is ordinarily accomplished by amphiphilic bilayers of a thickness of around 10 meters.
Researchers including Irene A. Chen and Jack W. Szostak have demonstrated that simple physicochemical properties of elementary protocells can give rise to simpler conceptual analogues of essential cellular behaviors, including primitive forms of Darwinian competition and energy storage. Such cooperative interactions between the membrane and encapsulated contents could greatly simplify the transition from replicating molecules to true cells. Competition for membrane molecules would favor stabilized membranes, suggesting a selective advantage for the evolution of cross-linked fatty acids and even the phospholipids of today. This micro-encapsulation allowed for metabolism within the membrane, exchange of small molecules and prevention of passage of large substances across it. The main advantages of encapsulation include increased solubility of the cargo and creating energy in the form of chemical gradients. Energy is thus often said to be stored by cells in molecular structures such as carbohydrates (including sugars), lipids, and proteins, which release energy when chemically combined with oxygen during cellular respiration.