Biology Reference
In-Depth Information
evolve towards another more ordered state. In so far as fluctuations
are inherent in any physical and chemical system (see chapter 2,
§2.2.4), this phenomenon has been likened to self-organisation. The
system is supposed to be capable of creating order spontaneously
without being influenced by an external cause. Prigogine called
these types of system “dissipative structures” in order to indicate
that the creation of order is accompanied by dissipation of energy.
The concept is used now to describe a large number of processes.
Adherents of self-organisation consider the origin and functioning of
living beings as relating to dissipative structures. We can quote the
two main examples of this given by Prigogine in his topic
Order Out
of Chaos
written in collaboration with Isabelle Stengers.
The appearance of the ordered movement of molecules forming
convection cells in a heated fluid, initially observed by Bénard
(1874-1939), is considered as the paradigm of self-organisation.
“(
…
)
Bénard instability is another striking example of the instability
of a stationary state giving rise to a phenomenon
of spontaneous
self-organisation
. The instability is due to a
vertical tempera-
ture gradient
27
set up in a horizontal liquid layer. The lower sur-
face of the latter is heated to a given temperature which is higher
than that of the upper surface. As a result of these boundary condi-
tions, a permanent heat flux is set up, moving from the bottom to
the top. When the
imposed gradient
27
reaches a threshold value,
the fluid's state of rest, the stationary state in which heat is con-
veyed by conduction alone, without convection, becomes unstable.
(…)
convection corresponding to the coherent motion of ensembles
of molecules is produced, increasing the rate of heat transfer.
Therefore, for given values of
the constraints (the gradient of
temperature)
,
27
the entropy production of the system is increased;
this contrasts with the theorem of minimum entropy.
(…)
Bénard
instability is a spectacular phenomenon. The convection motion
produced actually consists of the complex spatial organisation of
27
Original text not in bold.
