Biology Reference
In-Depth Information
models can account for the colored patterns over the surface of animals such as
leopards, zebra, and cats.
Prigogine suggested that the so-called far-from-equilibrium condition is both
necessary and sufficient for
self-organization
, but the general proof of this claim
may be lacking as already pointed out. Nevertheless, Prigogine and his group have
made important contributions to theoretical biology by establishing the concept that
structures in nature can be divided into two distinct classes -
equilibrium
and
dissipative structures
and that organisms are examples of the latter. It should be
noted that these two types of structures are not mutually exclusive, since many
dissipative structures (e.g., the living cell) require equilibrium structures as a part of
their components such as phospholipid bilayers of biomembranes (which last much
longer than, say, action potentials upon removing free energy supply).
One of the characteristic properties of all
self-organizing systems
is that the free
energy driving them is generated or produced within the system (concomitant to
self-organization), most often in the form of exergonic chemical reactions, either
catalyzed by enzymes (e.g., see Fig.
3.2
) or uncatalyzed (Fig.
3.1
). In contrast, there
are many organized systems that are driven by forces generated externally, such as
the Bernard instability (Prigogine 1980), which is driven by externally imposed
temperature gradients and paintings drawn by an artist's brush. To describe such
systems, it is necessary to have an antonym to “self-organization,” one possibility
of which being “other organization.” It is unfortunate that, most likely due to the
lack of the appropriate antonym, both self-organized (e.g., the flame of a candle)
and
other-organized
entities (e.g., a painting, or the Bernard instability) are lumped
together under the same name, that is,
self-organization
.
Dissipative structures are material systems that exhibit nonrandom behaviors in
space and/or time driven by irreversible processes. Living processes require both
equilibrium
and
dissipative structures
. Operationally, we may define the
equilib-
rium structures
of living systems as those structures that remain, and
dissipative
structures
as those that disappear, upon removing free energy input. Some
dissipa-
tive structures
can be generated from
equilibrium structures
through expenditure of
free energy, as exemplified by an acorn and a cold candle, both
equilibrium
structures
, turning into an oak and a flaming candle,
dissipative structures
, respec-
tively, upon input of free energy:
Free Energy
Equilibrium Structures
!
Dissipative Structures
(3.1)
The flame of a candle is a prototypical example of dissipative structures. The pattern
of colors characteristic of a candle flame reflects the space- and time-organized
oxidation-reduction reactions of hydrocarbons constituting the candle that produce
transient chemical intermediates, some of which emit photons as they undergo elec-
tronic transitions from excited states to ground states. From a mechanistic point of
view, the flame of a candle can be viewed as high-temperature self-organizing
chemi-
cal reaction-diffusion systems
in contrast to the Belousov-Zhabotinsky reaction
(Fig.
3.1
) which is a low-temperature self-organizing
chemical reaction-diffusion
system.
