Some Characteristics of Open Systems

We express this by saying that living systems are basically open systems (Burton, 1939; von Bertalanffy, 1940a; Chapter 5). An open system is defined as a system in exchange of matter with its environment, presenting import and export, building-up and breaking-down of its material components. Up to comparatively recent times physical chemistry, in kinetics and thermodynamics, was restricted to closed systems; the theory of open systems is relatively new and leaves many problems unsolved. The development of kinetic theory of open systems derives from two sources: first the biophysics of the living organism, secondly developments in industrial chemistry which, besides reactions in closed containers or batch processes, increasingly uses continuous reaction systems because of higher efficiency and other advantages. The thermodynamic theory of open systems is the so-called irreversible thermodynamics (Meixner & Reik, 1959); it became an important generalization of physical theory through the work of Meixner, Onsager, Prigogine and others.

Even simple open systems show remarkable characteristics (Chapter 5). Under certain conditions, open systems approach a time- independent state, the so-called steady state (Fliessgleichge- wicht after von Bertalanffy, 1942). The steady state is maintained in distance from true equilibrium and therefore is capable of doing work; as it is the case in living systems, in contrast to systems in equilibrium. The system remains constant in its composition, in spite of continuous irreversible processes, import and export, building-up and breaking-down, taking place. The steady state shows remarkable regulatory characteristics which become evident particularly in its equifinality. If a steady state is reached in an open system, it is independent of the initial conditions, and determined only by the system parameters, i.e., rates of reaction and transport. This is called equifinality as found in many organ- ismic processes, e.g., in growth (FIG. 6.1).

Fig. 6 .1 . Equifinality of growth. Heavy curve: normal growth of rats. Broken curve: at the 50th day, growth was stopped by vitamin deficiency. After reestablishment of normal regime, the animals reached the normal final weight. (After Hober from von Bertalanffy, 1960b).

In contrast to closed physico-chemical systems, the same final state can therefore be reached equifinally from different initial conditions and after disturbances of the process. Furthermore, the state of chemical equilibrium is independent of catalyzers accelerating the processes. The steady state, in contrast, depends on catalyzers present and their reaction constants. In open systems, phenomena of overshoot and false start (FIG. 6.2) may occur, with the system proceeding first in a direction opposite to that eventually leading to the steady state. Conversely, phenomena of overshoot and false start, as frequently found in physiology, may indicate that we are dealing with processes in open systems.

Fig. 6 .2 . Asymptotic approach to steady state (a) , false start (b) , and overshoot (c), in open systems. Schematic.

From the viewpoint of thermodynamics, open systems can maintain themselves in a state of high statistical improbability, of order and organization.

According to the second principle of thermodynamics, the general trend of physical processes is toward increasing entropy, i.e., states of increasing probability and decreasing order. Living systems maintain themselves in a state of high order and im- probability, or may even evolve toward increasing differentiation and organization as is the case in organismic development and evolution. The reason is given in the expanded entropy function of Prigogine. In a closed system, entropy always increases according to the Clausius equation:

dS ≥ 0                                                    (6.1)

In an open system, in contrast, the total change of entropy can be written according to Prigogine:

deS denoting the change of entropy by import, dtS the production of entropy due to irreversible processes in the system, such as chemical reactions, diffusion, heat transport, etc. The term dtS is always positive, according to the second principle; dS, entropy transport, may be positive or negative, the latter, e.g., by import of matter as potential carrier of free energy or “negative en­tropy.” This is the basis of the negentropic trend in organismic systems and of Schrodinger’s statement that “the organism feeds on negative entropy.”

More complex open-system models, approximating biological problems, have been developed and analyzed by Burton, Rashev- sky, Hearon, Reiner, Denbigh and other authors. In recent years, computerization has been widely applied for the solution of sets of numerous simultaneous equations (frequently nonlinear) (e.g., Franks, 1967; B. Hess and others) and for the simulation of complex open-system processes in physiological problems (e.g., Zerbst and coworkers; 1963 ff.). Compartment theory (Rescigno and Segre, 1967; Locker, 1966b) provides sophisticated methods for cases where reactions take place not in a homogenous space but in subsystems partly permeable to the reactants, as is the case in industrial systems and obviously many processes in the cell.

As can be seen, open systems compared with conventional closed systems show characteristics which seem to contradict the usual physical laws, and which were often considered as vitalistic characteristics of life, i.e., as a violation of physical laws, explainable only by introducing soul-like or entelechial factors into the organic happening. This is true of the equifinality of organic regulations, if, for example, the same “goal,” a normal organism, is produced by a normal, a divided, two fused ova, etc. In fact, this was the most important “proof of vitalism” according to Driesch. Similarly, the apparent contradiction of the trend toward increase of entropy and disorder in physical nature, and the negentropic trend in development and evolution were often used as vitalistic arguments. The apparent contradictions disappear with the expansion and generalization of physical theory to open systems.

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

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