4,388 research outputs found
Efficiency of autonomous soft nano-machines at maximum power
We consider nano-sized artificial or biological machines working in steady
state enforced by imposing non-equilibrium concentrations of solutes or by
applying external forces, torques or electric fields. For unicyclic and
strongly coupled multicyclic machines, efficiency at maximum power is not
bounded by the linear response value 1/2. For strong driving, it can even
approach the thermodynamic limit 1. Quite generally, such machines fall in
three different classes characterized, respectively, as "strong and efficient",
"strong and inefficient", and "balanced". For weakly coupled multicyclic
machines, efficiency at maximum power has lost any universality even in the
linear response regime
Compressive force generation by a bundle of living biofilaments
To study the compressional forces exerted by a bundle of living stiff
filaments pressing on a surface, akin to the case of an actin bundle in
filopodia structures, we have performed particulate Molecular Dynamics
simulations of a grafted bundle of parallel living (self-assembling) filaments,
in chemical equilibrium with a solution of their constitutive monomers.
Equilibrium is established as these filaments, grafted at one end to a wall of
the simulation box, grow at their chemically active free end and encounter the
opposite confining wall of the simulation box. Further growth of filaments
requires bending and thus energy, which automatically limit the populations of
longer filaments. The resulting filament sizes distribution and the force
exerted by the bundle on the obstacle are analyzed for different grafting
densities and different sub- or supercritical conditions, these properties
being compared with the predictions of the corresponding ideal confined bundle
model. In this analysis, non-ideal effects due to interactions between
filaments and confinement effects are singled out. For all state points
considered at the same temperature and at the same gap width between the two
surfaces, the force per filament exerted on the opposite wall appears to be a
function of a rescaled free monomer density . This
quantity can be estimated directly from the characteristic length of the
exponential filament size distribution observed in the size domain where
these grafted filaments are not in direct contact with the wall. We also
analyze the dynamics of the filament contour length fluctuations in terms of
effective polymerization () and depolymerization () rates, where again it
is possible to disentangle non-ideal and confinement effects.Comment: 24 pages, 7 figure
On the Properties of a Bundle of Flexible Actin Filaments in an Optical Trap
We establish the Statistical Mechanics framework for a bundle of Nf living
and uncrosslinked actin filaments in a supercritical solution of free monomers
pressing against a mobile wall. The filaments are anchored normally to a fixed
planar surface at one of their ends and, because of their limited flexibility,
they grow almost parallel to each other. Their growing ends hit a moving
obstacle, depicted as a second planar wall, parallel to the previous one and
subjected to a harmonic compressive force. The force constant is denoted as
trap strength while the distance between the two walls as trap length to make
contact with the experimental optical trap apparatus. For an ideal solution of
reactive filaments and free monomers at fixed free monomers chemical potential,
we obtain the general expression for the grand potential from which we derive
averages and distributions of relevant physical quantities, namely the obstacle
position, the bundle polymerization force and the number of filaments in direct
contact with the wall. The grafted living filaments are modeled as discrete
Wormlike chains, with Factin persistence length, subject to discrete contour
length variations to model single monomer (de)polymerization steps. Rigid
filaments, either isolated or in bundles, all provide average values of the
stalling force in agreement with Hill's predictions, independent of the average
trap length. Flexible filaments instead, for values of the trap strength
suitable to prevent their lateral escape, provide an average bundle force and
an average trap length slightly larger than the corresponding rigid cases (few
percents). Still the stalling force remains nearly independent on the average
trap length, but results from the product of two strongly L dependent
contributions: the fraction of touching filaments and the single filament
buckling force.Comment: 21 pages, 8 figure
Fertilizer Impact on Groundwater in Utah
Water quality has become the focal point of many decisions involving crop production. Crop production depends on specific inputs including fertilizer application. Without proper fertilization a farmer cannot achieve maximum economic returns. Crop yields in Utah have been increased over 50% by nitrogen fertilizer application alone. However, increasing nitrogen application beyond that needed for optimum economic return does more harm than good. This is especially true when groundwater concerns are ressed
The Phenomenology of a Top Quark Seesaw Model
The top quark seesaw mechanism offers a method for constructing a composite
Higgs field without the usual difficulties that accompany traditional
technicolor or topcolor theories. The focus of this article is to study the
phenomenology of the new physics required by this mechanism. After establishing
a set of criteria for a plausible top quark seesaw theory, we develop two
models, the first of which has a heavy weak singlet fermion with hypercharge
4/3 while the second has, in addition, a heavy weak singlet hypercharge -2/3
fermion. At low energies, these theories contain one or two Higgs doublets
respectively. We then derive the low energy effective Higgs potential in detail
for the two-doublet theory as well as study the likely experimental signatures
for both theories. A strong constraint on the one-doublet model is the measured
value of the rho parameter which permits the new heavy fermion to have a mass
of about 5-7 TeV, when the Higgs has a mass greater than 300 GeV. In the
two-doublet model, mixing of the new heavy Y=-2/3 fermion and the b quark
affects the prediction for R_b. In order to agree with the current limits on
R_b, the mass of this fermion should be at least 12 TeV. The mass of the heavy
Y=4/3 fermion in the two-doublet model is not as sharply constrained by
experiments and can be as light as 2.5 TeV.Comment: 33 pages, 12 figures, uses harvmac and picte
Modelling the species jump: towards assessing the risk of human infection from novel avian influenzas
The scientific understanding of the driving factors behind zoonotic and pandemic influenzas is hampered by complex interactions between viruses, animal hosts and humans. This complexity makes identifying influenza viruses of high zoonotic or pandemic risk, before they emerge from animal populations, extremely difficult and uncertain. As a first step towards assessing zoonotic risk of Influenza, we demonstrate a risk assessment framework to assess the relative likelihood of influenza A viruses, circulating in animal populations, making the species jump into humans. The intention is that such a risk assessment framework could assist decisionmakers to compare multiple influenza viruses for zoonotic potential and hence to develop appropriate strain-specific control measures. It also provides a first step towards showing proof of principle for an eventual pandemic risk model. We show that the spatial and temporal epidemiology is as important in assessing the risk of an influenza A species jump as understanding the innate molecular capability of the virus.We also demonstrate data deficiencies that need to be addressed in order to consistently combine both epidemiological and molecular virology data into a risk assessment framework
Fluctuation theorem and large deviation function for a solvable model of a molecular motor
We study a discrete stochastic model of a molecular motor. This discrete
model can be viewed as a \emph{minimal} ratchet model. We extend our previous
work on this model, by further investigating the constraints imposed by the
Fluctuation Theorem on the operation of a molecular motor far from equilibrium.
In this work, we show the connections between different formulations of the
Fluctuation Theorem. One formulation concerns the generating function of the
currents while another one concerns the corresponding large deviation function,
which we have calculated exactly for this model. A third formulation of FT
concerns the ratio of the probability of making one forward step to the
probability of making one backward step. The predictions of this last
formulation of the Fluctuation Theorem adapted to our model are in very good
agreement with the data of Carter and Cross [Nature, {\bf 435}, 308 (2005)] on
single molecule measurements with kinesin. Finally, we show that all the
formulations of FT can be understood from the notion of entropy production.Comment: 15 pages, 9 figure
The structure of electroweak corrections due to extended gauge symmetries
This paper studies models with extended electroweak gauge sectors of the form
SU(2) x SU(2) x U(1) x [SU(2) or U(1)]. We establish the general behavior of
corrections to precision electroweak observables in this class of theories and
connect our results to previous work on specific models whose electroweak
sectors are special cases of our extended group.Comment: 18 pages, 2 figures; added a referenc
Comparison of the sidereal angular velocity of subphotospheric layers and small bright coronal structures during the declining phase of solar cycle 23
Context. We compare solar differential rotation of subphotospheric layers
derived from local helioseismology analysis of GONG++ dopplergrams and the one
derived from tracing small bright coronal structures (SBCS) using EIT/SOHO
images for the period August 2001 - December 2006, which correspond to the
declining phase of solar cycle 23. Aims. The study aims to find a relationship
between the rotation of the SBCS and the subphotospheric angular velocity. The
northsouth asymmetries of both rotation velocity measurements are also
investigated. Methods. Subphotospheric differential rotation was derived using
ring-diagram analysis of GONG++ full-disk dopplergrams of 1 min cadence. The
coronal rotation was derived by using an automatic method to identify and track
the small bright coronal structures in EIT full-disk images of 6 hours cadence.
Results. We find that the SBCS rotate faster than the considered upper
subphotospheric layer (3Mm) by about 0.5 deg/day at the equator. This result
joins the results of several other magnetic features (sunspots, plages,
faculae, etc.) with a higher rotation than the solar plasma. The rotation rate
latitudinal gradients of the SBCS and the subphotospheric layers are very
similar. The SBCS motion shows an acceleration of about 0.005 deg/day/month
during the declining phase of solar cycle 23, whereas the angular velocity of
subsurface layers does not display any evident variation with time, except for
the well known torsional oscillation pattern. Finally, both subphotospheric and
coronal rotations of the southern hemisphere are predominantly larger than
those of the northern hemisphere. At latitudes where the north-south asymmetry
of the angular velocity increases (decreases) with activity for the SBCS, it
decreases (increases) for subphotospheric layers.Comment: 6pages, 8 figures, Accepted for publication in Astronomy and
Astrophysic
Heat shock protein 10 inhibits lipopolysaccharide-induced inflammatory mediator production
Heat shock protein 10 (Hsp10) and heat shock protein 160 (Hsp60) were originally described as essential mitochondrial proteins involved in protein folding. How,ever, both proteins have also been shown to have a number of extracellular immunomodulatory activities. Here we show that purified recombinant human Hsp10 incubated with cells in vitro reduced lipopolysaccharide (LPS)-induced nuclear factor-kappaB activation and secretion of several inflammatory mediators from RAW264.7 cells, murine macrophages, and human peripheral blood mononuclear cells. Induction of tolerance by contaminating LPS was formally excluded as being responsible for Hsp10 activity. Treatment of mice with Hsp10 before,endotoxin challenge resulted in the reduction of serum tumor necrosis factor-a and RANTES (regulated upon activation, normal T cell expressed and secreted) levels and an elevation of serum interleukin-10 levels. Hsp10 treatment also delayed mortality in a murine graft-ver-sus-host disease model, where gut-derived LPS contributes to pathology. We were unable to confirm previous reports that Hsp10 has tumor growth factor properties and suggest that Hsp10 exerts anti-inflammatory activity by inhibiting Toll-like receptor signaling possibly by interacting with extracellular Hsp60
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