Biology Reference
In-Depth Information
Ca ++ channels contains four of these repeats. Potassium channels are composed
of four subunits, each containing one repeat.
(p. 119)
Additional ion family channels are in the process of being discovered and
characterized, including a class of Cl channels. But already the number of
different channel types is according to Siegelbaum & Koester 'enormous'. The
diversity is accounted for in part because 'most channels are made up of multiple
subunits that can be combined in different permutations to produce channels
with different functional properties' (p. 119). Additionally, the variability is
'produced by differential expression of two or more closely related genes, by
alternative splicing of mRNA transcribed from the same gene, or by editing of
mRNA' (p. 120).
Some simplification of this extensive diversity occurs in the axonal region of
the neuron where just the two major channel types, Na + and K + , are involved.
However, even here, Hille also describes an extensive 'diversity of K channels'
in different tissues and even within single cells. He sums up this 'microhetero-
geneity of K channels', noting that 'such results are typical of experimental
discoveries today. The finer the method of analysis, the more apparent subtypes
of channels are discovered' (Hille, 2001, p. 74). In spite of this extensive diver-
sity and variation, genetics can provide a rationale for generalization, at least
involving similarity modeling. On this point Hille writes: 'The Na, Ca, and K
families of voltage-gated channels form a homologous gene superfamily, as may
be expected from their broad apparent functional similarity. This means that
many findings for one type of channel can be generalized to the others' (Hille,
2001; p. 85).
3.3. The H and H 'basic mechanism' as an emergent simplification
The account of the extensive diversity of specific mechanisms of ion chan-
nel types just summarized raises the question of how unity can be effectively
achieved amid such natural variation. In a significant sense, H and H achieved
that unification and simplification in advance of the more recent molecular
knowledge by working at a higher level of abstraction. Their accomplishment
suggests that in certain areas of biology, investigators can capture what might
be termed 'emergent simplifications' that transcend the specific workings of the
molecular details. In a way, a more abstract mechanism can be a 'basic mech-
anism', even if it is clearly realized that there are as yet unknown molecular
details of the mechanism. Possibly such a basic mechanism is more like a 'pro-
totypical' mechanism, which identifies and characterizes salient core features of
a biological entity and its actions.
Search WWH ::




Custom Search