16 - Energetic constraints on prebiotic pathways: application to the emergence of translation

Summary

The origin of life, as with any other process of structure formation, should have been accompanied by a loss of entropy. Since the second law of thermodynamics states that the entropy of an isolated system tends to increase, any self-organizing system must exchange free energy (closed system) and/or matter (open system) with its environment in order that the overall entropy increases (Kondepudi and Prigogine, 1998). This simple observation emphasizes the importance of energy transfers in the origin and development of early life. As far as biochemical systems are concerned, energy exchanges mostly involve chemical energy that is brought about by ‘high-energy’ carriers so that energy flows through metabolic pathways from free energy-rich compounds towards low-energy molecules, the difference being released in the environment as heat. When the occurrence of a thermodynamically unfavourable reaction makes it necessary, fresh energy is provided to the system through coupled reactions involving a free-energy carrier such as ATP. The principle that energy is brought about by ‘high-energy’ carriers applies to most metabolic pathways, though some of them do not simply follow this rule. An example is the process of energy collection leading to ATP synthesis, in which ‘chemical’ energy is generated from a ‘physico–chemical’ source: a gradient of concentration between two compartments separated by the plasma–cell membrane (Mitchell, 1961).