Temporally Asymmetric Fluctuations are Sufficient for the Biological Energy Transduction

A number of recent attempts to understand broad principles of energy transduction in biological systems have focused on correlation ratchets--systems which extract work out of fluctuations which are correlated in time [1-6]. Correlation ratchets are “information engines” analogous to Maxwell’s Demon, which extract work out of a bath by using information about the system to “choose” only those fluctuations which are helpful to make the engine run [8]. This information, which can only be acquired if the Demon is not in equilibrium with the bath [9], can be used to rectify the energy already available, but otherwise inaccessible in the thermal bath. Processes like this, in which the energy stored in a nonequilibrium bath is transformed into work at the expense of increased entropy, are believed to be the basis of “molecular motors,” and are of great importance in biology, and a number of other fields. It has been shown that the combination of a broken spatial symmetry in the potential (or ratchet potential) and time correlations in the driving are crucial, and enough to allow the transformation of the fluctuations into work [1]. The required broken spatial symmetry implies a specific molecular arrangement of the proteins involved. Here we show that a broken spatial symmetry is “not required,” and that temporally asymmetric fluctuations (with mean zero) can be used to do work, “even when the ratchet potential is completely symmetric.” Temporal asymmetry, defined as a lact of invariance of the statistical properties under the operation to temporal inversion, is a genetic property of nonequilibrium fluctuation, and should therefore be expected to be quite common in biological systems.