The Theory of Open Systems in Physics and Biology.

F ROM THE PHYSICAL POINT OF VIEW, the characteristic state of the living organism is that of an open system. A system is closed if no material enters or leaves it; it is open if there is import and export and, therefore, change of the components. Living systems are open systems, maintaining themselves in exchange of materials with environment, and in continuous building up and breaking down of their components. So far, physics and physical chemistry have been concerned almost exclusively with processes in closed reaction systems, leading to chemical equilibria. Chemical equilibria are found also in partial systems of the living organism-for example, the equilibrium between hemoglobin, oxyhemoglobin, and oxygen upon which oxygen transport by blood is based. The cell and the organism as a whole, however, do not comprise a closed system, and are never in true equilibrium, but in a steady state. We need, therefore, an extension and generalization of the principles of physics and physical chemistry, complementing the usual theory of reactions and equilibria in closed systems, and dealing with open systems, their steady states, and the principles governing them. Though it is usual to speak of the organism as a "dynamic equilibrium," only in recent years has theoretical and experimental investigation of open systems and steady states begun. The conception of the organism as an open system has been advanced by von Bertalanffy since 1932, and general kinetic principles and their biological implications have been developed (4, 6). In German literature, Dehlinger and Wertz (15), Bavink (1), Skrabal (31), and others have extended these conceptions. A basically similar treatment was given by Burton (12). The paper of Reiner and Spiegelman (28) seems to have been inspired by conversations of the present author with Reiner in 1937-38. Starting from problems of technological chemistry, the comparison of efficiency in batch and continuous reaction systems, Denbigh (16) has also developed the kinetics of open reaction systems. The most important recent work is the thermodynamics of open systems by Prigogine (25, 26). In physics, the theory of open systems leads to fundamentally new principles. It is indeed the more general theory, the restriction of kinetics and thermodynamics to closed systems concerning only a rather special case. In biology, it first of all accounts for many characteristics of living systems that have appeared to be in contradiction to the laws of physics, and have been considered hitherto as vitalistic features. Second, the consideration of organisms as open systems yields quantitative laws of important biological phenomena. So far, the consequences of the theory have been developed especially in respect to biological problems, but the concept will be important for other fields too, such as industrial chemistry and meteorology.