Biology Reference
In-Depth Information
Thermodynamic considerations apply to all machines, not just biological
ones. All must operate within an energy flow between a high-energy source
and a low-energy sink and must draw upon the energy available at the source
and release its waste products, now in a lower energy state, into the sink.
A waterwheel, in which water at a higher elevation stores more free energy
than the water at a lower elevation, often provides a fruitful metaphor for this
process. What is critical for any mechanism is that the energy liberated in this
flow is employed to perform work and this requires that it be channeled in
the appropriate way. This typically means that it is transformed into a different
format - the waterwheel converts the energy stored in water at a higher elevation
into the rotational energy of the wheel's axle, and linkages appended to the axle
in turn convert the motion into the form required.
What is distinctive in the case of living organisms is that the thermodynamics
must be regulated to enable the organism to maintain itself - to repair itself
or to build itself initially. This need is brought into clear focus by considering
the autocatalytic networks, to which theorists such as Stuart Kauffman (1995)
have turned in their accounts of the origin of life. Such networks, as well as
other self-organizing systems such as hurricanes and tornadoes, are extremely
fragile and are not able to maintain themselves for long. Part of the reason is that
they rely on an energy source which may be quickly expended. But a further
part of the reason is that they are not organized to channel the energy they
secure to construct themselves so as to extract more energy from the source in
the future (Ruiz-Mirazo & Moreno, 2004; see also Bickhard, 1993; Kauffman,
2000). (This would be pointless if there is no further energy source to tap, but
of great importance if there is a continued source of energy, but one that can be
utilized only if the system is properly configured.)
Whereas Varela did not focus on the thermodynamics and the management
of energy in his account of autonomy, Kepa Ruiz-Mirazo and Alvaro Moreno
(2004) have made it central to their account. They begin with the recognition
that as organized systems, living systems are far from thermodynamic equi-
librium and, in order to maintain that organization, must maintain themselves
far from equilibrium (cf. Schrödinger, 1944). Many of the chemical reactions
required to maintain such a system are endergonic (require Gibbs free energy)
and so must be coupled with those that liberate energy from another source
(exergonic reactions). In order to maintain themselves far from equilibrium,
Moreno focuses on how the system manages the flow of energy so as to provide
for its own construction and reconstruction. The membrane presents one point
the bonds that render them into solids make them less subject to dissipation than structures in fluid milieus.
On the other hand, it is harder to design a self-repair process for a system made of solids, which may explain
why the strategy for dealing with breakdown in human-engineered systems has been to build in redundancy
rather than self-repair.
