The Force Exerted by a Molecular Motor

The stochastic driving force exerted by a single molecular motor (e.g., a kinesin, or myosin) moving on a periodic molecular track (microtubule, actin filament, etc.) is discussed from a general viewpoint open to experimental test. An elementary "barometric" relation for the driving force is introduced that (i) applies to a range of kinetic and stochastic models, (ii) is consistent with more elaborate expressions entailing explicit representations of externally applied loads and, (iii) sufficiently close to thermal equilibrium, satisfies an Einstein-type relation in terms of the velocity and diffusion coefficient of the (load-free) motor. Even in the simplest two-state models, the velocity-vs.-load plots exhibit a variety of contrasting shapes (including nonmonotonic behavior). Previously suggested bounds on the driving force are shown to be inapplicable in general by analyzing discrete jump models with waiting time distributions.Comment: submitted to PNA

Extended Kinetic Models with Waiting-Time Distributions: Exact Results

Inspired by the need for effective stochastic models to describe the complex behavior of biological motor proteins that move on linear tracks exact results are derived for the velocity and dispersion of simple linear sequential models (or one-dimensional random walks) with general waiting-time distributions. The concept of ``mechanicity'' is introduced in order to conveniently quantify departures from simple ``chemical,'' kinetic rate processes, and its significance is briefly indicated. The results are extended to more elaborate models that have finite side-branches and include death processes (to represent the detachment of a motor from the track).Comment: 17 pages, 2 figure

Constituency campaigning at the 2015 general election

The context of the 2015 general election suggested that the electoral impact of parties’ constituency campaigns could vary as a consequence in particular of the relative unpopularity of the Liberal Democrats. Using data from a survey of election agents, this paper analyses how the main GB level political parties adapted the intensity of their constituency level campaign’s to ensure that to varying degrees they produced positive electoral payoffs. It further analyses the electoral effects of face-to-face campaigning and e-campaigning at constituency level and shows that while e-campaigning has grown in importance, face-to-face campaigning continues to deliver stronger electoral benefits. Overall, the 2015 election illustrated that intense constituency level campaigning continues to be electorally beneficial for all the parties, but that this was the election when the Conservative Party became genuinely effective in terms of the delivery of electoral payoffs.This research was funded by the Economic and Social Research Council (Grant No. ES/M007251/1

Central Star Formation in Pseudobulges and Classical Bulges

I use Spitzer 3.6-8.0 \mu m color profiles to compare the radial structure of star formation in pseudobulges and classical bulges. Pseudobulges are ``bulges'' which form through secular evolution, rather than mergers. In this study, pseudobulges are identified using the presence of disk-like structure in the center of the galaxy (nuclear spiral, nuclear bar, and/or high ellipticity in bulge); classical bulges are those galaxy bulges with smooth isophotes which are round compared to the outer disk, and show no disky structure in their bulge. I show that galaxies structurally identified as having pseudobulges have higher central star formation rates than those of classical bulges. Further, I also show that galaxies identified as having classical bulges have remarkably regular star formation profiles. The color profiles of galaxies with classical bulges show a star forming outer disk with a sharp change, consistent with a decline in star formation rates, toward the center of the galaxy. Classical bulges have a nearly constant inner profile (r < 1.5 kpc) that is similar to elliptical galaxies. Pseudobulges in general show no such transition in star formation properties from the outer disk to the central pseudobulge. Thus I conclude that pseudobulges and classical bulges do in fact form their stars via different mechanisms. Further, this adds to the evidence that classical bulges form most of their stars in fast episodic bursts, in a similar fashion to elliptical galaxies; whereas, pseudobulges form stars from longer lasting secular processes.Comment: accepted to ApJ Letter

Coexistence Curve Singularities at Critical End Points

We report an extensive Monte Carlo study of critical end point behaviour in a symmetrical binary fluid mixture. On the basis of general scaling arguments, singular behaviour is predicted in the diameter of the liquid-gas coexistence curve as the critical end point is approached. The simulation results show clear evidence for this singularity, as well as confirming a previously predicted singularity in the coexistence chemical potential. Both singularities should be detectable experimentally.Comment: 9 pages Revtex, 3 figures. To appear in Phys. Rev. Let

Critical behavior of the three-dimensional bond-diluted Ising spin glass: Finite-size scaling functions and Universality

We study the three-dimensional (3D) bond-diluted Edwards-Anderson (EA) model with binary interactions at a bond occupation of 45% by Monte Carlo (MC) simulations. Using an efficient cluster MC algorithm we are able to determine the universal finite-size scaling (FSS) functions and the critical exponents with high statistical accuracy. We observe small corrections to scaling for the measured observables. The critical quantities and the FSS functions indicate clearly that the bond-diluted model for dilutions above the critical dilution p*, at which a spin glass (SG) phase appears, lies in the same universality class as the 3D undiluted EA model with binary interactions. A comparison with the FSS functions of the 3D site-diluted EA model with Gaussian interactions at a site occupation of 62.5% gives very strong evidence for the universality of the SG transition in the 3D EA model.Comment: Revised version. 10 pages, 9 figures, 2 table