Open Systems and Steady States in Biology

Any modern investigation of metabolism and growth has to take into account that the living organism as well as its components are so-called open systems, i.e., systems maintaining themselves in a continuous exchange of matter with environment (FIG. 7.1). The essential point is that open systems are beyond the limits of conventional physical chemistry in its two main branches, kinetics and thermodynamics. In other terms, conventional kinetics and thermodynamics are not applicable to many processes in the living organism; for biophysics—the application of physics to the living organism—an expansion of theory is necessary.

Fig. 1.1. a: Model of a simple open system, showing maintenance of constant concentrations in the steady state, equifinality, adaptation and stimulus-response, etc. The model can be interpreted as a simplified schema for protein synthesis (A: amino acids, B: protein, C: deamination products; k^ polymerization of amino acids into protein, k2: depolymeri­zation, /¡8: deamination; k2 <K A energy supply for protein synthesis not indicated) . In somewhat modified form, the model is Sprinson 8c Ritten- berg’s (1949) for calculation of protein turnover from isotope experi­ments. (After von Bertalanffy, 1953a) .

b: The open system of reaction cycles of photosynthesis in algae. (After Bradley & Calvin, 1957)

The living cell and organism is not a static pattern or machinelike structure consisting of more or less permanent “building materials” in which “energy-yielding materials” from nutrition are broken down to provide the energy requirements for life processes. It is a continuous process in which both so-called building materials as well as energy-yielding substances (Bau- and Betriebsstoffe of classical physiology) are broken down and regenerated. But this continuous decay and synthesis is so regulated that the cell and organism are maintained approximately constant in a so-called steady state (Fliessgleichgewicht, von Bertalanffy). This is one fundamental mystery of living systems; all other characteristics such as metabolism, growth, development, self-regulation, reproduction, stimulus-response,     autonomous       activity,         etc.,      are        ultimately consequences of this basic fact. The organism’s being an “open system” is now acknowledged as one of the most fundamental criteria of living systems, at least so far as German science is concerned (e.g., von Bertalanffy, 1942; Zeiger, 1955; Butenandt 1955, 1959).

Before going further, I wish to apologize to the German col- leagues for dwelling on matters which are familiar to them, and which I myself have often presented. As Dost (1962a) stated in a recent paper, “our sons already in their premedical examination take account of this matter,” i.e., of the theory of open systems in their kinetic and thermodynamic formulations. Remember— to quote but two examples—the presentation of the topic by Blasius (1962) in the new editions of our classic Landois- Rosemann textbook, and Netter in his monumental Theoretical Biochemistry (1959). I am sorry to say that the same does not apply to biophysics and physiology in the United States. I have looked in vain into leading American texts even to find the terms, “open system,” “steady state” and “irreversible thermodynamics.” That is to say, precisely that criterion which fundamentally distinguishes living systems from conventional inorganic ones is generally ignored or bypassed.

Consideration of the living organisms as an open system  ex-changing matter with environment comprises two questions: first, their statics, i.e., maintenance of the system in a time-independent state; secondly, their dynamics, i.e., changes of the system in time.

The problem can be considered from the viewpoints of kinetics and of thermodynamics.

Detailed discussion of the theory of open systems can be found in the literature (extensive bibliographies in von Bertalanffy 1953a, 1960b). So I shall restrict myself to saying that such systems have remarkable features of which I will mention only a few. One fundamental difference is that closed systems must eventually attain a time-independent state of chemical and thermodynamic equilibrium; in contrast, open systems may attain, under certain conditions, a time-independent state which is called a steady state, Fliessgleichgewicht, using a term which I introduced some twenty years ago. In the steady state, the composition of the system remains constant in spite of continuous exchange of components. Steady states or Fliessgleichgewichte are equifinal (FIG. 6.1); i.e., the same time-independent state may be reached from different initial conditions and in different ways—much in contrast to conventional physical systems where the equilibrium state is determined by the initial conditions. Thus even the simplest open reaction systems show that characteristic which defines biological restitution, regeneration, etc. Furthermore, classical thermodynamics, by definition, is only concerned with closed systems, which do not exchange matter with environment. In order to deal with open systems, an expansion and generalization was necessary which is known as irreversible thermodynamics. One of its consequences is elucidation of an old vitalistic puzzle. According to the second principle of thermodynamics, the general direction of physical events is toward states of maximum entropy, probability and molecular disorder, levelling down existing differentiations. In contrast and “violent contradiction” to the second principle (Adams, 1920), living organisms maintain themselves in a fantastically improbable state, preserve their order in spite of continuous irreversible processes and even proceed, in embryonic development and evolution, toward ever higher differentiations. This apparent riddle disappears by the consideration that the classic second principle by definition pertains only to closed systems. In open systems with intake of matter rich in high energy, maintenance of a high degree of order and even advancement toward higher order is thermodynamically permitted.

Living systems are maintained in a more or less rapid exchange, degeneration and regeneration, catabolism and anabolism of their components. The living organism is a hierarchical order of open systems. What imposes as an enduring structure at a certain level, in fact, is maintained by continuous exchange of components of the next lower level. Thus, the multicellular organism maintains itself in and by the exchange of cells, the cell in the exchange of cell structures, these in the exchange of composing chemical com- pounds, etc. As a general rule, turnover rates are the faster the smaller the components envisaged (Tables 6.1-3). This is a good illustration for the Heraclitean flow in and by which the living organism is maintained.

So much about the statics of open systems. If we take a look at changes of open systems in time, we also find remarkable characteristics. Such changes may occur because the living system initially is in an unstable state and tends toward a steady state; such are, roughly speaking, the phenomena of growth and development. Or else, the steady state may be disturbed by a change in external conditions, a so-called stimulus; and this—again roughly speaking— comprises adaptation and stimulus-response. Here too characteristic differences to closed systems obtain. Closed systems generally tend toward equilibrium states in an asymptotic approach. In contrast, in open systems, phenomena of false start and overshoot may occur (FIG. 6.2). In other terms: If we find overshoot or false start—as is the case in many physiological phenomena—we may expect this to be a process in an open system with certain predictable mathematical characteristics.

As a review of recent work (Chapter 6) shows, the theory of the organism as an open system is a vividly developing field as it should be, considering the basic nature of biological Fliessgleichgewicht. The above examples are given because, after the basic investigations by Schonheimer (1947) and his group into the “Dynamic State of Body Constituents” by way of isotope tracers, the field is strangely neglected in American biology which, under the influence of cybernetic concepts, rather has returned to the machine concept of the cell and organism, thereby neglecting the important principles offered by the theory of open systems.

Source: Bertalanffy Ludwig Von (1969), General System Theory: Foundations, Development, Applications, George Braziller Inc.; Revised edition.

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