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Many organisms consist of modules, both
anatomically and in their metabolism. Anatomical modules are usually
segments or organs. When we look at illustrations of metabolic
reactions, we find that they, too, are modular: we can clearly
identify, for instance, the citric acid cycle as a complex
network that has only a few interfaces with other such modules.
This principle holds true at various different scales: we can
identify smaller modules within such larger networks that are
similarly self-contained. We say that metabolic modularity is scale-free.
In addition to showing scalefree and small world properties, biological networks
appear to exhibit modularity in topological
structure. In the field of network biology, the definition of
nodes and edges in a given
network depends on the type of network examined. For example, in a
protein interaction network, nodes correspond with
individual proteins and edges represent the interactions between
them (either through direct physical interaction, or
compound-mediated). Metabolic networks, on the other
hand, contain metabolite nodes and edges that represent
the specific enzymes that connect them (in catalyzing biochemical
reactions). As with any type of network, modularity in biological
networks allows sub-groups of nodes and edges to function in a
semi-autonomous fashion.
The concept of modularity resurfaces at the scale of organs and
developmental units. Why are there distinct cell types organised
into spatial aggregations (organs), and what are the benefits of
having a segmented body plan, containing
different modules (for instance, thoracic and abdominal segments in
an arthropod) and where one of the possible differences between
species is in the number of each type of module they possess?
Interestingly, this property has led researchers to suggest that
modularity imparts a certain degree of evolvability to a system by allowing
specific features (i.e. network sub-groups) to undergo changes
without substantially altering the functionality of the entire
system. Essentially, each module is free to evolve within, so long
as the interfaces between modules remain consistent. This would
suggest that the metabolic pathways at the edges between modules
are more constrained. It is thought that there exists an optimal
degree of modularity for each given organism.
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