An example metabolic subnetwork. (A): A small metabolic network with 17 reactions. Metabolites are shown as nodes, while reactions are shown by arrows. Reactions 1, 8, 9, 15 and 16 are boundary reactions, while all other reactions are internal reactions. We might be interested only in a subnetwork containing nine reactions: 1, . . ., 9, which are shown by thick arrows. This subnetwork will be called SuN. (B): The reduced subsystem comprising only the nine interesting reactions.