Physical chemistry presents the theory of kinetics and equilibria in chemical systems. As example, consider the reversible reaction in ester formation:
C2H5OH + CH3 • COOH <=> CH3COO • C2H5 + H2O,
in which always a certain quantitative ratio between alcohol and acetic acid on the one hand, and between ester and water on the other, is established.
Application of physico-chemical equilibrium principles, especially of chemical kinetics and the law of mass action, has proved to be of fundamental importance for the explanation of physiological processes. An example is the function of blood, to transport oxygen from the lung to the tissues of the body and, conversely, carbon dioxide formed in the tissues to the lungs for exhalation; the process results from the equilibria between hemoglobin, oxyhemoglobin and oxygen according to the law of mass action, and quantitative formulations can be stated not only for the simple conditions in hemoglobin solution, but also for the more complicated ones in the blood of vertebrates. The importance of kinetic consideration of enzyme reactions, of respiration, fermentation, etc., is well known. Similarly, other physico-chemical equilibria (distribution, diffusion, adsorption, electrostatic equi-libria) are of fundamental physiological significance (cf. Moser and Moser-Egg, 1934).
Considering the organism as a whole, it shows characteristics similar to those of systems in equilibrium (cf. Zwaardemaker, 1906, 1927). We find, in the cell and in the multicellular organism, a certain composition, a constant ratio of the components, which at first resembles the distribution of components in a chemical system in equilibrium and which, to a large extent, is maintained under different conditions, after disturbances, at different body size, etc.: an independence of composition of the absolute quantity of components, regulative capacity after disturbances, constancy of composition under changing conditions and with changing nutrition, etc. (cf. von Bertalanffy, 1932, pp. 190ff.; 1937, pp. 80ff.).
We realize at once, however, that there may be systems in equilibrium in the organism, but that the organism as such cannot be considered as an equilibrium system.
The organism is not a closed, but an open system. We term a system “closed” if no material enters or leaves it; it is called “open” if there is import and export of material.
There is, therefore, a fundamental contrast between chemical equilibria and the metabolizing organisms. The organism is not a static system closed to the outside and always containing the identical components; it is an open system in a (quasi-)steady state, maintained constant in its mass relations in a continuous change of component material and energies, in which material continually enters from, and leaves into, the outside environment.
The character of the organism as a system in steady (or rather quasi-steady) state is one of its primary criteria. In a general way, the fundamental phenomena of life can be considered as consequences of this fact. Considering the organism over a shorter span of time, it appears as a configuration maintained in a steady state by the exchange of components. This corresponds to the first main field of general physiology—i.e., physiology of metabolism in its chemical and energetic aspects. Superimposed on the steady state are smaller process waves, basically of two kinds. First there are periodic processes originating in the system itself and hence autonomic (e.g., automatic movements of the organs of respiration, circulation and digestion; automatic-rhythmic, electrical activities of nerve centers and the brain supposedly resulting from rhythmic chemical discharges; automatic move- merits of the organism as a whole). Secondly, the organism reacts to temporary changes in environment, to “stimuli,” with reversible fluctuations of its steady state. This is the group of processes caused by changes of external conditions and hence heteronomic subsumed in physiology of excitation. They can be considered as temporary disturbances of the steady state from which the organism returns to “equilibrium,” to the equal flow of the steady state. Such consideration has proved to be useful and leading to quantitative formulations (cf. p. 137). Finally, the definition of the state of the organism as steady state is valid only in first approximation, insofar as we envisage shorter periods of time in an “adult” organism, as we do, for example, in investigating metabolism. If we take the total life cycle, the process is not stationary but only quasi-stationary, subject to changes slow enough to abstract from them for certain research purposes, and comprising embryonic development, growth, aging, death, etc. These phenomena, not quite exhaustively encompassed under the term of morphogenesis, represent the third large complex of problems in general physiology. Such consideration proves especially useful in areas accessible to quantitative formulation.
In general, physical chemistry is limited almost exclusively to consideration of processes in closed systems. To these refer the well- known formulations of physical chemistry; the law of mass action, in particular, is used only for definition of true chemical equilibria in closed systems. The applicability of chemical equilibria to, e.g., transfer reactions is based on the fact that these are fast ionic reactions attaining equilibrium. Open chemical systems are hardly taken into consideration in physical chemistry. This restriction of kinetics to closed systems is understandable; for open systems are more difficult to establish technically, and not of major importance in the purely physical consideration. Nevertheless, such arrangements are easily visualizable—e.g., when in a reaction a 
b the product b of the left-to- right reaction is continually removed from the system by suitable means (precipitation, dialysis through a membrane permeable only for b but not for a, etc.) while a is continually introduced into the system. Systems of this kind occasionally occur in technological chemistry; continuous fermentation in the production of acetic acid is an example for what is here called “open chemical system.”
However, such systems are of great importance to the biologist.
For open chemical systems are indeed realized in nature in the form of living organisms, maintaining themselves in a continuous exchange of their components. “Life is a dynamic equilibrium in a polyphasic system” (Hopkins).
We therefore need a definition of the so-called stationary equi- librium, the constancy of composition in the change of components, similarly as well-known expressions of physical chemistry define true chemical equilibria in closed systems.
Obviously, the reaction system and reaction conditions are infinitely more complicated in organisms than in the systems usually dealt with in physical chemistry. These are reactions among an extraordinarily high number of components. Moreover, the cell and organism are not homogeneous systems (a true solution), but represent highly heterogeneous, colloidal systems so that reactions depend not only on mass action but on many physico-chemical factors of adsorption, diffusion, etc. Even enzyme reactions in the test tube do not, in general, simply follow the law of mass action. This being the case, it is clear that reactions even in simple organismic systems cannot be written in a closed system of equations; this is possible only for isolated partial systems. It is, however, possible, first, to state certain general principles for open systems, irrespective of the special nature of the system. Secondly, although in view of the enormous number of reactions in the organism and even the individual cell, it is impossible to follow individual reactions, expressions can be used that represent statistical averages of a multitude of incalculable or even unknown processes. Such a procedure is already applied in chemistry by using overall formulas for reactions consisting of numerous steps. Similarly, balance equations in physiology of metabolism and bioenergetics are based on statistical averages resulting from numerous and largely unknown processes in intermediary metabolism. We may, for instance, summarize anabolic and catabolic processes as “assimilation” and “dissimilation,” respectively, and consider, as a first approximation, the steady state as balance of “assimilation” and “dissimilation.” Such magnitudes, representing statistical averages of a multitude of inextricable processes, can be used for calculation in a way similar to that conventionally used in physical chemistry for individual compounds and reactions.
The maintenance of the system in a continuous flow and exchange of material and energy, the order of innumerable physicochemical reactions in a cell or organism in a way granting the first, the maintenance of a constant ratio of the components even under different conditions, after disturbances, at different sizes, etc., are the central problems of organic metabolism. The doublefaced change of living systems in assimilation and dissimilation manifests—in the words of von Tschermaks (1916)—a trend toward maintenance of a certain state, regeneration compensating the disturbance caused by degeneration. How is it that what has been lost in the process is rebuilt from the materials offered in nutrition, that building blocks liberated by enzymes find the right place in the organismic system so that it maintains itself in metabolism? What is the principle of “automatic self-regulation” of metabolism? We are possessed of a vast knowledge of physicochemical processes in the cell and in the organism; but we must not overlook the fact “that even after complete explanation of individual processes, we are worlds away from fully understanding the total metabolism of a cell” (M. Hartmann, 1927, p. 258). Ex- tremely little is known about the principles controlling the individual processes in the way indicated above. No wonder that again and again the problem led to vitalistic conclusions (e.g. Kottje, 1927).
Obviously, general principles as those we are going to develop cannot provide a detailed explanation of those problems; they can, however, indicate the general physical foundations of that essential characteristic of life, self-regulation of metabolism and maintenance in change of components. The special way in which these are realized in individual metabolic processes can be determined only by experimental investigation. It can be hoped, however, that the general consideration alerts to possibilities hitherto hardly envisaged, and that the formulations proposed, or similar equations, be apt to describe concrete individual phenomena.
Source: Bertalanffy Ludwig Von (1969), General System Theory: Foundations, Development, Applications, George Braziller Inc.; Revised edition.