Biology Reference
In-Depth Information
3. AUTONOMY OF MATERIAL SYSTEMS: THE NEED FOR
SPECIFIC CATALYSIS
A logic of life, at least of earth-bound life as we know it, can be deduced from
two basic postulates:
Postulate 1. Living organisms are material objects.
Postulate 2. Living organisms are autonomous.
The first postulate commits us to a view of life that is inextricably linked
to chemistry: the science of spontaneous transformation of matter and therefore
the science of creativity and what Stuart Kauffman (2000) calls the 'adja-
cent possible'. The creative nature of chemistry is captured in the concept of
'component systems' (Kampis, 1991). Whether a chemical transformation will
actually occur under specified conditions, and if it does, how fast, is answered
from thermodynamic and kinetic considerations. An important generalisation
of chemical biology is that covalent chemistry is virtually exclusively enzyme-
catalysed, whereas the noncovalent chemistry involved in, for instance, chemical
recognition, protein folding and self-assembly of macromolecular complexes is
largely uncatalysed (although we now know that at least folding is often assisted
by chaperones). This distinction between molecular (covalent) and supramolec-
ular (noncovalent) chemistry, made by Jean-Marie Lehn (1995), will be seen
further on to be crucial in understanding the ability of living cells to fabricate
themselves. Supramolecular chemistry refers to the formation of ordered molecu-
lar aggregates that are held together by noncovalent binding interactions. Because
these forces are weak, the formation of supramolecular assemblies is usually ther-
modynamically controlled and therefore a spontaneous process of self-assembly
rather than a sequential bond-forming synthesis.
A recent series of papers (Ruiz-Mirazo & Moreno, 2004, 1998, Ruiz-Mirazo
et al., 1998, 2004) provide an excellent analysis of the concept of autonomy, not
only as a point of departure for a universal definition of life, but also in relation
to autopoietic theory (see also Chapter 11). They make a strong and convincing
argument that the concept of autonomy is multifaceted; living systems exemplify
all these facets, whereas the autopoietic perspective only considers an abstract
organisation that recursively produces itself; real-world autonomy cannot escape
the requirements of chemistry, energetics and kinetics, and the necessity for
spatial autonomy by self-bounding.
9
Living systems are open and can never be fully thermodynamically
autonomous; as dissipative structures they depend on an externally determined
9
However, Ruiz-Mirazo and Moreno emphasise the thermodynamic aspects of autonomy, and virtually ignore
the kinetic aspects, which, in my opinion, are just as, if not more, important.
