Biology Reference
In-Depth Information
1
Control Systems in the Living World
The Nature of Living Systems
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 (
Figure 1.1
).
We viscerally distinguish between a mountain as a nonliving entity and a tree as a
living thing. The key difference is the biological pattern of the tree, which is repeated
regularly. Matterhorn is unique, naturally unrepeatable, and there is no Matterhorn
pattern. This unmistakable biopattern is unique to living things.
But, ill defined as it is, the “biopattern” may not always be a reliable indication of
life, and most will agree that the pictures below may defy our perception of biopat-
tern and animal pattern. At first glance, one may not notice the difference between a
stick insect and a dead twig (
Figure 1.2
) or a living one (
Figure 1.3
), but our doubts
disappear as soon as we see them move or react to our touch. Our gut instinct, again,
is that only living things are irritable and display motile avoidance behavior. Besides,
humans also always knew that living things grow and reproduce.
Aristotle ('Aριστοτέλης, 384-322 bc) used a similar empirical approach 23 cen-
turies ago to develop his classification of the living world, which was based on visual
perceptions of living things and phenomena. He believed both living and nonliving
things
exist
and are distinguished by the
soul
, which is found only in living organ-
isms. Aristotle characterized the plants as living entities with souls, which enables
them to
grow
and
reproduce
; in addition, an evolved type of soul allows animals to
perceive
the external world,
move
, and react to it instinctively, while the “human
soul” enables us to do everything animals do, plus
use logic
and
think
.
Since Aristotle, biology has made considerable progress in attempting to know
the structure and function of living systems and understand their essential proper-
ties behind the visual perception. Two centuries after the discovery of the cell by the
English scientist Robert Hooke (1635-1703), zoologist Theodor Schwann (1810-
1882) and biologist Matthias Jakob Schleiden (1804-1881) suggested that cells
were the basic units of all living beings, unicellulars and multicellulars, animals and
plants. This great generalization had a profound heuristic effect on biological studies
and represents a landmark in the development of biology and in its gradual transfor-
mation from a descriptive into a causal science.
The progress in the study of the cell and living systems in general has created a
detailed picture of high organization and functional complexity. Life is a process that
biological systems have to perform in order to maintain and to perpetuate, via repro-
duction, their highly improbable structures. Life is an inseparable manifestation of
