Biology Reference
In-Depth Information
laws, if they exist, appear to be subject to spatial limitations, are never without
exception, and have more often than not non-deducible cause-effect relation-
ships. Some philosophers even completely reject the idea that biology has laws.
In contrast, Shulman in his contribution claims that there is no need to abandon
the philosophy of physics in biology, but his point of view seems to be an excep-
tion in this topic. And Westerhoff and Kell pointed out that systems biology
does have general, nomological laws, which are valid generally provided that
the premises are defined as formally as is done in physics. After all, Boyle's
law is never valid either, not even for Helium.
One concept of biological laws is linked to the unificationist idea of what
counts as an explanation. The central dogma of molecular biology, i.e., infor-
mation flows from DNA to RNA to protein, was considered by many to be a
biological law that was global (but perhaps not universal), exceptionless, and
necessary until the discovery of the retroviruses. Thereafter the old law was
restricted, and the 'new law' claims that information flows from nucleic acids
to protein, which has not been challenged seriously so far. This example seems
prototypical of the fate of 'laws' in biology, but one may wonder if this is a
problem at all. Generalizations with only limited applicability over a certain
biological domain are still worthwhile to pursue. The theorems of metabolic
control analysis are good examples in this respect. The conceptual dogma of the
rate-limiting step (see Fell, this volume), which reigned sovereign for so many
years in biochemistry textbooks, has been incorporated into the systems biolog-
ical, much subtler notion of distributed control in biochemical pathways, where
the rate-limiting step is still an explicit possibility, albeit it is acknowledged now
that it only occurs in exceptional cases. Several additional useful theorems have
been derived afterwards, although they all have limited applicability. The theo-
rems of metabolic control analysis (see Fell; Shulman; Westerhoff & Kell, this
volume) are also relevant with respect to the question whether the unificationist,
the causal/mechanical, and the nomological modes of explanation can be linked
in a useful manner. Biochemical pathways and their constituent enzymes are
clear examples of a part-whole system that is composed of two adjacent levels
of cellular organization, both of which feature in the quantitative mechanistic
explanation. For, the 'local' role of each enzyme within the pathway and its
level of control on the global steady-state pathway flux can be articulated in a
quantitative way. At the same time, however, a systemic law can be formulated
for the same part-whole system, i.e., the sum of all the flux control coefficients
of the pathway should be equal to one. This result could be generalized to any
arbitrary pathway, and it could therefore well be granted the status of a law.
Also to more complicated nonlinear pathways - for example, pathways including
branches and cycles - the same law applies. Thus metabolic control analysis
enables a straightforward linkage of the two mainstream ideas about scientific
explanations, the causal/mechanical and the unificationist type of explanation.
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