The Nature of Living Systems

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Although biologists are still arguing about the nature and definition of life, humans have always been able to distinguish intuitively between living and nonliving things. It is obvious that the Matterhorn mountain is inanimate, while the tree alongside it is alive

Living Systems Have Clear-Cut Boundaries
Living systems build clear boundaries that separate them from the environment, thus determining the range of action or “territoriality” of the organism’s homeostatic mechanisms. On the inner side of the boundary is the system; on the outer is its surrounding. In unicellulars, this boundary is represented by the cell membrane. In multicellulars, it is represented by skin/integument (animals) or bark (plants). Living cells build boundaries to control the flow of matter and energy rather than isolate
themselves from the environment. The cell membrane is an integral part of the living system. It is designed to allow for the controlled intake of nutrients and the excretion of waste and nonusable energy (heat), while preventing the free diffusion of solutes that is necessary to maintain differences in concentration between the cell and its environment. It represents the front line for the antientropic drama of the living system, to vanquish thermodynamic forces of disorder and to build, maintain, and perpetuate its physically improbable structure.

Metabolism
In performing their vital functions, living systems obtain energy by breaking down nutrients, depleting their reserves of matter and free energy (catabolism). In order to maintain their structure and function, living systems have to compensate for the loss by synthesizing the lost components through the nutrients they take in with food (anabolism). The equilibrium between the catabolism and anabolism in living systems represents their normal metabolism, which enables them to maintain a state of dynamic material and energetic equilibrium.

Since the seventeenth century, metabolism has been considered a defining property of living systems (viruses are metabolic parasites, and it is the host cell that goes astray to produce its own killers). The maintenance of the structure and functions of the cell require spatiotemporal coordination of a multitude of anabolic and catabolic reactions occurring in the living cell. But the maintenance of the naturally eroding cell structure requires that the cell somehow “knows” or has information on the
structure to be maintained and does the species-specific work at the right places and at the right times within the cell’s nanospace.

How does the cell accomplish this? If metabolism is understood to be the work that the cell does to retain its structural identity, then how does the cell get the information on the changes occurring in the system, how does it detect the deviations from the norm, and, finally, how does it generate instructions to restore the normal structure and send them to the changed structures? But if the living cell is not controlled by external forces, as is clearly the case, metabolism implies the presence of a
built-in control system.


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