What is life? The single difference between living and non-living things (which every biologist should know)

Until now, lack of a satisfactory definition of life has been the elephant in the room for biologists. Despite devoting our studies to understanding life, we teach our students that we cannot define it but must describe life using a list of characteristics. Indeed, biology is a fantastically intricate subject that is difficult to reduce to a single distinguishing feature, or a single difference between 'living' and 'non-living' things. However, armed with modern technology (especially high-speed atomic force microscopy) and an understanding of physics we can directly observe matter operating inside living things across scales from atoms and single molecules up to the scale of cells, and see what happens in real time. This has revealed a single fundamental process shared by disparate structures operating within cells: the direction of thermodynamic disequilibria into unidirectional motion by molecules sharing the operating principle of conformation change along one plane. Unidirectional motion (linear or rotational) is a characteristic of life processes underpinning the mechanism of heredity and metabolic functions, with self-regulating and replicating networks of thermally-agitated 'uniplanar' molecules consolidating structure from sparse resources (Schrödinger's 'negative entropy'). This allows us to define life, to define a living thing, and also non-life and death. As this is a simple physical principle rather than being based on a specific molecule or chemical interaction, such 'directed thermodynamics' life processes are likely to be widespread across the universe. Relevance to ALife research is also discussed. Based on a peer-reviewed paper (Pierce S. Life’s Mechanism. Life. 2023; 13(8):1750. https://doi.org/10.3390/life13081750) and discussed in a video presentation (www.youtube.com/watch?v=WVQjFvR1Qa0).