Before revision:  Fluctuations in such a system are not only generated intrinsically by molecular dynamics, but also extrinsically, by interactions of the system with the rest of the network and its environment. Concentration fluctuations of a substance outside the studied system can enter it through a nonlinear uptake reaction which acts as a nonlinear filter.
After revision:  Fluctuations in such a system : intrinsic noise, inherent in the studied reactions, and extrinsic noise generated in other parts of the network or in the environment. We study the effect of extrinsic noise entering the system through a nonlinear uptake reaction which acts as a nonlinear filter.