Biology Reference
In-Depth Information
increase in the concentration of
Ac
and
I
at point
k
(Fig. 3B). But,
Ac
and
I
diffuse in space: because
I
diffuses more quickly than
Ac
,
its concentration increases more rapidly to the right and left of
k
(Fig. 3C). The inhibitor effect of
I
on the synthesis of
Ac
therefore
comes to dominate in these regions and, consequently, the con-
centration of
Ac
decreases while
I
continues to accumulate there
(Fig. 3D). This process continues until the system arrives at a new
state of equilibrium characterised by alternating peaks of concen-
trations of
Ac
and
I
(Fig. 3E). Therefore in the end the system
moves from a homogeneous to a heterogeneous state. According to
Turing, this concentration heterogeneity of
Ac
and
I
is the starting
point for morphological differentiation of the embryo, owing to the
local differences it implies in the properties of the system.
In this metastable system there is certainly a random event, the
fluctuation, which triggers the reaction-diffusion mechanism, but,
once it has been produced, the dynamics of the system are totally
deterministic. They depend on the specific relationships between
the constituents,
Ac
and
I
, and are defined by a system of equations
without a random variable.
The concept of noise is also the basis for the theories of self-
organisation. These theories state that, as in reaction-diffusion
mechanisms, if the disturbances affecting a system exceed a certain
threshold, instead of returning to its state of equilibrium the sys-
tem will evolve towards another more complex state known as an
attractor state. New characteristics will then emerge which cause
qualitative changes. Those favouring these theories have suggested
several different mechanisms to account for these emergence phe-
nomena (see chapter 5).
To illustrate now the case of an intrinsically random mecha-
nism, let us return to the example of the car. It has to do the same
journey as the one demonstrating determinism with noise, but its
speed, instead of being constant, varies in a totally random fashion
all the time.
12
If the journey is repeated several times, the car will
12
The speed is continually determined randomly. In view of technological progress
it would be possible to construct such a crazy car.
