Biology Reference
In-Depth Information
Gibbs energy gradient (Nicolis & Prigogine, 1977). However, living systems
also create internal non-equilibrium conditions that allow them a degree of ther-
modynamic autonomy. As an example, consider a chemotrophic bacterium that
not only grows on glucose by fermenting or oxidising it, but also stores glucose
as glycogen. If the external glucose is depleted, i.e. if the external Gibbs energy
gradient collapses, this bacterium will still be able to survive due to the internal
nonequilibrium condition that it has created. As long as its glycogen store lasts
it is thermodynamically autonomous with regard to its carbon source. There is,
therefore, a difference between a dissipative system in which a certain range of
external conditions create and maintain the system (so that if outside this range
the dissipative system no longer exists), and an autonomous dissipative sys-
tem that also actively creates and maintains internal nonequilibrium conditions.
A Bénard cell would be an example of the first type, a living cell an example
of the latter.
To be kinetically autonomous, the chemical reactions that comprise the system
must operate on a faster timescale than the rest of the underlying network of
spontaneous mass-action chemical transformations; the greater the separation
on the timescale, the smaller the effects of these spontaneous side-reactions
and the greater the degree of kinetic autonomy. This can only be achieved by
catalysts that are specific with regard to both reactants/products and reaction;
kinetic autonomy therefore absolutely requires the existence of catalysts that
specifically recognise their substrates and transform them into specific products.
If such catalysts are themselves short-lived, the autonomous system must be
able to replace them. In short, such a system must itself also be a catalyst
factory. However, to fabricate molecular catalysts requires both building blocks
and additional machinery, which itself must be made within the system. The
building blocks can of course be supplied by the environment, but even if the
system has to fabricate them this is not a problem; all it needs is to be able
to make the specific catalysts that will accomplish the synthesis. However, the
machinery that constructs the catalysts must itself be replaceable by the system,
lest it fails; this implies even more additional machinery. It is clearly here that
the linear hierarchy of efficient causes followed up to now seems to wander off
into an infinite regress that is incompatible with the existence of real autonomous
systems. In some way, this hierarchy of efficient causation must fold back into
itself, must close, must become circular.
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The possibility, mentioned above, for internal creation and maintenance
of nonequilibrium conditions and their dynamic adaptation in the face of
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The kinetic autonomy that is ensured by specific catalysts is essentially what is lacking from Tibor Ganti's
chemoton (Gánti, 2003). There is nothing in a chemoton that would prevent its chemical intermediates
dissipating into side-reactions. Much closer to the kinetic autonomy of living systems is that of the autocatalytic
Belousov-Zhabotinsky reaction system (Fiel & Burger, 1985) in which the catalytic species are produced
within the system itself.
