Abstract
Ideas of homeostasis derive from the concept of the organism as an open system. These ideas can be traced back to Heraclitus. Hopkins, Bernard, Hill, Cannon, Weiner and von Bertalanffy developed further the mechanistic basis of turnover of biological components, and Schoenheimer and Rittenberg were pioneers of experimental approaches to the problems of measuring pool sizes and dynamic fluxes. From the second half of the twentieth century, a biophysical theory mainly founded on self-organisation and Dynamic Systems Theory allowed us to approach the quantitative and qualitative analysis of the organised complexity that characterises living systems. This combination of theoretical framework and more refined experimental techniques revealed that feedback control of steady states is a mode of operation that, although providing stability, is only one of many modes and may be the exception rather than the rule. The concept of homeodynamics that we introduce here offers a radically new and all-embracing concept that departs from the classical homeostatic idea that emphasises the stability of the internal milieu toward perturbation. Indeed, biological systems are homeody- namic because of their ability to dynamically self-organise at bifurcation points of their behaviour where they lose stability. Consequently, they exhibit diverse behaviour; in addition to monotonic stationary states, living systems display complex behaviour with all its emergent characteristics, i.e., bistable switches, thresholds, waves, gradients, mutual entrainment, and periodic as well as chaotic behaviour, as evidenced in cellular phenomena such as dynamic (supra)molecular organisation and flux coordination. These processes may proceed on different spatial scales, as well as across time scales, from the very rapid processes within and between molecules in membranes to the slow time scales of evolutionary change. It is dynamic organisation under homeodynamic conditions that make possible the organised complexity of life.
Highlights
In the second half of the last century, fundamental information about the spatio-temporal organisation in living systems became established
Selforganisation is deeply rooted in nonequilibrium thermodynamics[5] and the kinetics of nonlinear systems whereas Dynamic Systems Theory lies within the geometric theory of dynamical systems created by Poincaré[6,7]
The concept of dynamic organisation has been introduced to describe function in cellular, or even supracellular, systems that arises as the spatio-temporal coherence of events resulting from the intrinsic, autonomous, dynamics of biological processes[9].The concept of homeostasis refers to the relative constancy, i.e., stable steady states, of the internal milieus physiological status
Summary
In the second half of the last century, fundamental information about the spatio-temporal organisation in living systems became established. Selforganisation is deeply rooted in nonequilibrium thermodynamics[5] and the kinetics of nonlinear systems whereas Dynamic Systems Theory lies within the geometric theory of dynamical systems created by Poincaré[6,7]. By applying this biophysical theory of biological organisation to successively more complicated systems (i.e., artificial, artificial-biological-oriented, or biological),[8,9] it became clear that self-organisation is a fundamental and necessary property of living systems. The type (i.e., negative or positive) and number of feedbacks nested in these mass-energy-information-carrying networks determine the dynamic behaviour of living systems by providing the necessary nonlinearities and coupling between processes. These emergent properties are crucial in the transfer and processing of information in biochemical as well as cellular networks[12,13] that may be modulated either by genetic or epigenetic mechanisms
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