  Perturbations are ubiquitous in metabolism. A central tool to understand and
control their influence on metabolic networks is sensitivity analysis, which
investigates how the network responds to external perturbations. We follow here
a structural approach: the analysis is based on the network stoichiometry only
and it does not require any quantitative knowledge of the reaction rates. We
consider perturbations of reaction rates and metabolite concentrations, at
equilibrium, and we investigate the responses in the network. For general
metabolic systems, this paper focuses on the sign of the responses, i.e.
whether a response is positive, negative or whether its sign depends on the
parameters of the system. In particular, we identify and describe the
subnetworks that are the main players in the sign description. These
subnetworks are associated to certain kernel vectors of the stoichiometric
matrix and are thus independent from the chosen kinetics.
