  The living cell uses a variety of molecular receptors to read and process
chemical signals that vary in space and time. We model the dynamics of such
molecular level measurements as Markov processes in steady state, with a
coupling between the receptor and the signal. We prove exactly that, when the
the signal dynamics is not perturbed by the receptors, the free energy consumed
by the measurement process is lower bounded by a quantity proportional to the
mutual information. Our result is completely independent of the receptor
architecture and dependent on signal properties alone, and therefore holds as a
general principle for molecular information processing.
