Control Systems in the Living World - Building the Most Complex Structure on Earth - page 2

Figure 1.1 The Matterhorn in the Swiss Alps and a tree.

Figure 1.3 A stick insect on a living

Figure 1.2 A stick insect on a dead branch.

twig.

the existence of living systems, and all living systems, from a unicellular prokaryote
to a human being, have in common several essential properties.

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 sur-
rounding. In unicellulars, this boundary is represented by the cell membrane. In mul-
ticellulars, 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 sys-
tem, to vanquish thermodynamic forces of disorder and to build, maintain, and per-
petuate its physically improbable structure.