  We study the effect of extrinsic noise in metabolic networks. We introduce
external random fluctuations at the kinetic level, and show how these lead to a
stochastic generalization of standard Metabolic Control Analysis. While
Summation and Connectivity Theorems hold true in presence of extrinsic noise,
control coefficients incorporate its effect through an explicit dependency on
the noise intensity. New elasticities and response coefficients are also
defined. Accordingly, the concept of control by noise is introduced as a way of
tuning the systemic behaviour of metabolisms. We argue that this framework
holds for intrinsic noise too, when time-scale separation is present in the
system.
