  In living cells, chemical reactions are connected by sharing their products
and substrates, and form complex networks, e.g. metabolic pathways. Here we
developed a theory to predict the sensitivity, i.e. the responses of
concentrations and fluxes to perturbations of enzymes, from network structure
alone. Responses turn out to exhibit two characteristic patterns,
$localization$ and $hierarchy$. We present a general theorem connecting
sensitivity with network topology that explains these characteristic patterns.
Our results imply that network topology is an origin of biological robustness.
Finally, we suggest a strategy to determine real networks from experimental
measurements.
