  Understanding cell function requires an accurate description of how noise is
transmitted through biochemical networks. We present an analytical result for
the power spectrum of the output signal of a biochemical network that takes
into account the correlations between the noise in the input signal (the
extrinsic noise) and the noise in the reactions that constitute the network
(the intrinsic noise). These correlations arise from the fact that the
reactions by which biochemical signals are detected necessarily affect the
signaling molecules and the detection components of the network simultaneously.
We show that anti-correlation between the extrinsic and intrinsic noise
enhances the robustness of zero-order ultrasensitive networks to biochemical
noise. We discuss the consequences of the correlation between extrinsic and
intrinsic noise for a modular description of noise transmission through large
biochemical networks in the context of the mitogen-activated protein kinase
cascade.
