Life's mechanism

The multifarious internal workings of biological organisms are difficult to reconcile with a single feature defining a state of ‘being alive’. Indeed, theories and definitions of life typically rely on sets of emergent properties, such as growth, capacity to evolve and apparent purpose (agency), that are only symptomatic of intrinsic functioning. Crucially, empirical studies demonstrate that biological molecules such as ratcheting/rotating enzymes (motor proteins), ribozymes and chlorophyll undergo repetitive conformation state changes driven by thermal agitation and/or energy exchanges. These molecules exhibit disparate structures and roles, but all govern biological processes relying on directional physical motion (e.g., DNA transcription, translation, ATP synthesis, cytoskeleton transport, photosynthetic resonance energy transfer). Fundamentally, they share the mechanistic principle of repetitive conformation changes in one plane (uniplanar) driven by thermodynamic gradients, producing dependable unidirectional motion: all are ‘heat engines’ using thermodynamic disequilibria to perform work. Recognition that disparate biological molecules share a heat engine principle governing directional motion, working in self-regulating networks, allows a robust, mechanistic definition of life: life is a self-regulating process whereby matter undergoes cyclic, uniplanar conformation state changes that convert thermodynamic disequilibria into directed motion, performing work that locally reduces entropy. A ‘living thing’ is thus a structure comprising, at least in part, an autonomous network of units operating on the heat engine principle. Death is loss of integrated heat engine function. The principle of self-regulating networks of heat engines is independent of any specific chemical environment, and could be applied to other biospheres.