Thermodynamic cost of molecular computation

Living systems update their status by altering the probability distribution of stochastically rearranging molecules in response to a change in the system parameters. These updates constitute molecular computational steps. Due to the presence of thermodynamic driving forces, typically in the form of chemical gradients, these computations convert a molecular system from one non-equilibrium steady state to another. Because such steady states are energetically costly to maintain, the question arises as to why nature has evolved this computational scheme. I will discuss a thermodynamic limit on computation. Namely, for any molecular system performing any computational step, the maximum and minimum information gained in the computation is shown to be a simple function of the thermodynamic force. Therefore, the presence of thermodynamic forces, and the expenditure of energy, allows biomolecular systems to convert modest changes in input into striking changes in output that would be surprising or impossible at equilibrium.Numerical sampling demonstrates the tightness of this universal limit, and as examples, we show that Ras signaling and microtubule assembly can closely approach it.