More models of muscle movement

Quantum transport in a periodic symmetric potential of a driven quantum system

A system-reservoir nonlinear coupling model has been proposed to the situation when the system is driven externally by a random force and the associated bath is kept in thermal equilibrium, in an attempt to put forth a microscopic approach to quantum state-dependent diffusion and multiplicative noises in terms of a quantum Langevin equation in the overdamped limit (quantum Smoluchowski equation). We then obtain the analytical expression for phase induced quantum current in a periodic potential when the external noise has finite correlation time and explore the dependence of the current on various parameters related to the external noise, for example, the noise strength.

Transport of overdamped Brownian particles in a two-dimensional tube: nonadiabatic regime

Transport of overdamped Brownian particles in a two-dimensional asymmetric tube is investigated in the presence of nonadiabatic periodic driving forces. By using Brownian dynamics simulations we can find that the phenomena in nonadiabatic regime differ from that in adiabatic case. The direction of the current can be reversed by tuning the driving frequency. Remarkably, the current as a function of the driving amplitude exhibits several local maxima at finite driving frequency.

Phase induced current in presence of nonequilibrium bath: A quantum approach

Based on a system-reservoir nonlinear coupling model, where the associated bath is externally driven by a fluctuating force, we present a microscopic approach to quantum state-dependent diffusion and multiplicative noises in terms of a quantum (Markovian) Langevin equation in overdamped limit when the associated bath is in nonequilibrium state. We then explore the possibility of observing a quantum current when the bath is modulated by white noise, the phenomena which is absent in the classical regime.

Directed motion in a periodic potential of a quantum system coupled to a heat bath driven by a colored noise

A system-reservoir nonlinear coupling model is proposed for a quantum system when the associated bath is not in thermal equilibrium but is modulated by an external colored noise, to present a microscopic approach to quantum state-dependent diffusion and multiplicative noise in terms of a quantum Langevin description. Consequently, the Fokker-Planck equation in position space, valid in the overdamped limit, for multiplicative colored noise is constructed to explore the possibility of observing a quantum current and dependence of the current on various parameters of external noise is examined.

Transport driven by spatially modulated noise in a periodic tube

This paper investigates a three-dimensional periodic tube driven by spatially modulated Gaussian white noise. We derive an analytical expression for the net current by introducing entropic barriers. It is found that the phase shift between the entirely symmetric tube and noise modulation can break the symmetry of the generalized potential and induce directed transport. The sign of the current is determined by the phase shift. The current is a peaked function of the bottleneck radius. The interplay between the asymmetric tube and noise modulation can also induce a net current.

Thermal noise can facilitate energy transformation in the presence of entropic barriers

Efficiency of a Brownian particle moving along the axis of a three-dimensional asymmetric periodic channel is investigated in the presence of a symmetric unbiased force and a load. Reduction of the spatial dimensionality from two or three physical dimensions to an effective one-dimensional system entails the appearance of entropic barriers and an effective diffusion coefficient. The energetics in the presence of entropic barriers exhibits peculiar behavior which is different from that occurring through energy barriers. We found that even on the quasistatic limit there is a regime where the efficiency can be a peaked function of temperature, which indicates that thermal noise can facilitate energy transformation, contrary to the case of energy barriers. The appearance of entropic barriers may induce optimized efficiency at a finite temperature.

Transport reversal in a thermal ratchet

Transport of a Brownian particle moving in a periodic potential is investigated in the presence of a symmetric unbiased external force. The viscous medium is alternately in contact with two heat reservoirs. We present the analytical expression of the net current at the quasi-steady-state limit. It is found that the competition of the asymmetric parameter of the potential with the temperature difference leads to phenomena like current reversal. The competition between the two driving factors is a necessary but not a sufficient condition for current reversals.

Ratchet behavior in nonlinear Klein-Gordon systems with pointlike inhomogeneities

We investigate the ratchet dynamics of nonlinear Klein-Gordon kinks in a periodic, asymmetric lattice of pointlike inhomogeneities. We explain the underlying rectification mechanism within a collective coordinate framework, which shows that such a system behaves as a rocking ratchet for point particles. Careful attention is given to the kink width dynamics and its role in the transport. We also analyze the robustness of our kink rocking ratchet in the presence of noise. We show that the noise activates unidirectional motion in a parameter range where such motion is not observed in the noiseless case. This is subsequently corroborated by the collective variable theory. An explanation for this phenomenon is given.

Nanometres and piconewtons: the macromolecular mechanics of kinesin

Much of our current understanding of the molecular physiology of kinesin has come from in vitro motility assays: indeed, the discovery of kinesin relied upon such assays. By marrying in vitro assays with novel instruments capable of resolving movements on the molecular scale, it has proved possible to make measurements on single motors. Such key parameters as the step size, stepping force, and force-velocity relationship for kinesin have been determined in this fashion, and should soon contribute to a molecular model for the movement of kinesin.

Fluctuations-induced switch in the gene transcriptional regulatory system

Based on the kinetic model of genetic regulation system proposed by Am. J. Physiol. 274, c531 (1998)], the effects of fluctuations in the degradation reaction rate and the synthesis reaction rate of the transcription factor have been investigated through numerical computation and analysis theory. In the case of uncorrelated noises, it is shown that only the fluctuation of degradation reaction rate can induce a switch process, and the mean first passage time (MFPT) from the high concentration state to the low concentration one is decreased when the noise intensity of degradation reaction rate is increased. In the case of correlations between noises, a switch process can also be induced by the cross-correlation intensity between noises and by the fluctuation of the synthesis reaction rate in the genetic regulatory system. It is found that, under large cross-correlation intensity, a successive switch process (i.e., "on" --> "off" --> "on," which we call the reentrance transition or twice switch ) occurs with an increase of noise intensities, and a critical noise intensity exists at which the MFPT of the switch process is the largest. While the system is initially in the high concentration state with an increase of the cross correlation, the stationary probability distribution (SPD) of the transcription factor activator monomer concentration at the low concentration state is increased, yet the MFPT is increased due to the decreasing of the SPD of the transient states between the two steady stable states.

Multiple current reversals in forced inhomogeneous ratchets

Transport properties of overdamped Brownian particles in a rocked thermal ratchet with space dependent friction coefficient is studied. By tuning the parameters, the direction of current exhibit multiple reversals, both as a function of the thermal noise strength as well as the amplitude of rocking force. Current reversals also occur under deterministic conditions and exhibit intriguing structure. All these results arise due to mutual interplay between potential asymmetry, noise, driving frequency, and inhomogeneous friction.

Josephson vortex in a ratchet potential: theory

We propose a type of Josephson vortex ratchet. In this system a Josephson vortex moves in a periodic asymmetric potential under the action of a deterministic or random force with zero time average. For some implementations the amplitude of the potential can be controlled during the experiment, thus allowing us to tune the performance of the system and build rocking as well as flashing ratchets. We discuss the differences between conventional and Josephson vortex ratchets and present a model describing the dynamics of the fluxon in such a system. We show numerical simulation results that predict rectification of a monochromatic, deterministic signal with zero time average. The investigation of this system may lead to the development of the fluxon rectifier-a device that produces a dc voltage from nonequilibrium fluctuations.

Disorder induced diffusive transport in ratchets

The effects of quenched disorder on the overdamped motion of a driven particle on a periodic, asymmetric potential are studied. While for the unperturbed potential the transport is due to a regular drift, the quenched disorder induces a significant additional chaotic "diffusive" motion. Possible applications to experiments in nanoscale surfaces and particle separation are discussed.

Active unfolding of precursor proteins during mitochondrial protein import

Precursor proteins made in the cytoplasm must be in an unfolded conformation during import into mitochondria. Some precursor proteins have tightly folded domains but are imported faster than they unfold spontaneously, implying that mitochondria can unfold proteins. We measured the import rates of artificial precursors containing presequences of varying length fused to either mouse dihydrofolate reductase or bacterial barnase, and found that unfolding of a precursor at the mitochondrial surface is dramatically accelerated when its presequence is long enough to span both membranes and to interact with mhsp70 in the mitochondrial matrix. If the presequence is too short, import is slow but can be strongly accelerated by urea-induced unfolding, suggesting that import of these 'short' precursors is limited by spontaneous unfolding at the mitochondrial surface. With precursors that have sufficiently long presequences, unfolding by the inner membrane import machinery can be orders of magnitude faster than spontaneous unfolding, suggesting that mhsp70 can act as an ATP-driven force-generating motor during protein import.