Living systems approached from physical principles.

This article attempts to review our work in the field since 2008, attempts to put it in a coherent framework and takes a courageous look vis-à-vis the bigger picture. It summarizes our approach, successes and open questions to start from physical principles when approaching living systems. It stresses the importance of conservation laws versus material and/or structural approaches to living systems commonly taken in (molecular) biology. Indeed, we claim that the crucial system in biology isn't a molecule or a molecular class whatsoever, but the interface created by biomolecules in water. It is the physical or thermodynamic state of this 2D interface and the action of conservation laws on it, which determines biological function, an approach I refer to as the "state-to-function-approach" in stark contrast to the structure-function approach.Three key ideas, all based on physical principles, particularly the 2nd law of thermodynamics and momentum conservation, are presented and experimentally confirmed. In Idea One we bridge physical state and biological function directly, e.g. by demonstrating the control of enzymatic activity and ion conductivity via thermodynamic state. Idea Two presents the role of momentum conservation in biological communication specifically applied to the principles of nerve pulse propagation. Idea Three finally introduces a physical concept of specificity, which is free of structural requirements and includs the specific interaction between pulses and enzymes.We finally discuss the extend of applicability and the universality of the mentioned ideas by presenting some impressive similarities between fairly remotely appearing biological processes, such as cell growth and pulse propagation. We close with the question in how far a thermodynamic approach can bring insight in the concept of cell adaptation, the evolution of organs or a deeper understanding of health and disease?