Proton flows, proton gradients and subcellular architecture in biological energy conversion

Hydrogen cations, or protons, provide a medium by which energy is stored in biological systems and converted for use in metabolic transformations. Such pre-eminence relies on the interplay between interfacial and bulk or chemical transformations, according to mechanisms that are shared by organisms in all phyla of life. The present work describes the fundamental aspects of biological energy management by focusing on the relationship between vectorial proton flows and the geometry of energy producing organelles in eukaryotes. The two leading models of proton-mediated energy conversion, the delocalised proton (or chemiosmotic) model and the localised proton model, are presented in a complementary perspective. While the delocalised model provides a description that relies on equilibrium thermodynamics, the localised model addresses dynamic processes that are better described using out-of-equilibrium thermodynamics. The work reviews the salient aspects of such mechanisms, traces the development of our present understanding, and highlights the areas that are open to future developments.