611 research outputs found
Spontaneous Oscillations of Collective Molecular Motors
We analyze a simple stochastic model to describe motor molecules which
cooperate in large groups and present a physical mechanism which can lead to
oscillatory motion if the motors are elastically coupled to their environment.
Beyond a critical fuel concentration, the non-moving state of the system
becomes unstable with respect to a mode with angular frequency omega. We
present a perturbative description of the system near the instability and
demonstrate that oscillation frequencies are determined by the typical
timescales of the motors.Comment: 11 pages, Revtex, 4 pages Figure
SCOTCH and SODA: A Transformer Video Shadow Detection Framework
Shadows in videos are difficult to detect because of the large shadow
deformation between frames. In this work, we argue that accounting for shadow
deformation is essential when designing a video shadow detection method. To
this end, we introduce the shadow deformation attention trajectory (SODA), a
new type of video self-attention module, specially designed to handle the large
shadow deformations in videos. Moreover, we present a new shadow contrastive
learning mechanism (SCOTCH) which aims at guiding the network to learn a
unified shadow representation from massive positive shadow pairs across
different videos. We demonstrate empirically the effectiveness of our two
contributions in an ablation study. Furthermore, we show that SCOTCH and SODA
significantly outperforms existing techniques for video shadow detection. Code
is available at the project page:
https://lihaoliu-cambridge.github.io/scotch_and_soda/Comment: Accepted to CVPR 202
Increasing the uptake of HIV testing to reduce undiagnosed infection and prevent transmission among black African communities living in England: Barriers to HIV testing
Globally, the HIV epidemic continues to have an impact on the lives of millions of
people. In 2008, there were an estimated 83,000 people living with HIV (both
diagnosed and undiagnosed), equivalent to 1.3 per 1000 population in the UK. In
that same year, 7,798 people were newly diagnosed with HIV. The global epidemic is
reflected in the UK; around 38% (2,790) of these newly diagnosed infections were
among black Africans who acquired their HIV through heterosexual contact. It is
thought that most (87%) of these infections among black Africans in the UK were
acquired abroad, mainly in sub-Saharan Africa (Health Protection Agency 2009).
Late diagnosis of HIV is defined as diagnosis taking place after anti-retroviral
treatment would normally have begun, or when the person has an illness which
defines them as having AIDS. It is the most important factor associated with HIVrelated
disease and death in the UK and is a particular problem among black
Africans. In 2007, over 40% of new diagnoses among black Africans were classified
as ‘late’.
HIV testing can help reduce transmission of the virus. People who find out they have
HIV may change their sexual behaviour as a result of the diagnosis. A negative HIV
test provides an opportunity for preventive education and advice and may also lead
to changes in behaviour. Increasing the frequency of testing may result in earlier
detection of HIV following infection - when it is most virulent - providing greater
opportunity to reduce transmissio
Active Membrane Fluctuations Studied by Micropipet Aspiration
We present a detailed analysis of the micropipet experiments recently
reported in J-B. Manneville et al., Phys. Rev. Lett. 82, 4356--4359 (1999),
including a derivation of the expected behaviour of the membrane tension as a
function of the areal strain in the case of an active membrane, i.e.,
containing a nonequilibrium noise source. We give a general expression, which
takes into account the effect of active centers both directly on the membrane,
and on the embedding fluid dynamics, keeping track of the coupling between the
density of active centers and the membrane curvature. The data of the
micropipet experiments are well reproduced by the new expressions. In
particular, we show that a natural choice of the parameters quantifying the
strength of the active noise explains both the large amplitude of the observed
effects and its remarkable insensitivity to the active-center density in the
investigated range. [Submitted to Phys Rev E, 22 March 2001]Comment: 14 pages, 5 encapsulated Postscript figure
Two-Component Fluid Membranes Near Repulsive Walls: Linearized Hydrodynamics of Equilibrium and Non-equilibrium States
We study the linearized hydrodynamics of a two-component fluid membrane near
a repulsive wall, via a model which incorporates curvature- concentration
coupling as well as hydrodynamic interactions. This model is a simplified
version of a recently proposed one [J.-B. Manneville et al. Phys. Rev. E, 64,
021908 (2001)] for non-equilibrium force-centres embedded in fluid membranes,
such as light-activated bacteriorhodopsin pumps incorporated in phospholipid
(EPC) bilayers. The pump/membrane system is modeled as an impermeable,
two-component bilayer fluid membrane in the presence of an ambient solvent, in
which one component, representing active pumps, is described in terms of force
dipoles displaced with respect to the bilayer midpoint. We first discuss the
case in which such pumps are rendered inactive, computing the mode structure in
the bulk as well as the modification of hydrodynamic properties by the presence
of a nearby wall. We then discuss the fluctuations and mode structure in steady
state of active two-component membranes near a repulsive wall. We find that
proximity to the wall smoothens membrane height fluctuations in the stable
regime, resulting in a logarithmic scaling of the roughness even for initially
tensionless membranes. This explicitly non-equilibrium result, a consequence of
the incorporation of curvature-concentration coupling in our treatment, also
indicates that earlier scaling arguments which obtained an increase in the
roughness of active membranes near repulsive walls may need to be reevaluated.Comment: 39 page Latex file, 3 encapsulated Postscript figure
Energy Transduction of Isothermal Ratchets: Generic Aspects and Specific Examples Close to and Far from Equilibrium
We study the energetics of isothermal ratchets which are driven by a chemical
reaction between two states and operate in contact with a single heat bath of
constant temperature. We discuss generic aspects of energy transduction such as
Onsager relations in the linear response regime as well as the efficiency and
dissipation close to and far from equilibrium. In the linear response regime
where the system operates reversibly the efficiency is in general nonzero.
Studying the properties for specific examples of energy landscapes and
transitions, we observe in the linear response regime that the efficiency can
have a maximum as a function of temperature. Far from equilibrium in the fully
irreversible regime, we find a maximum of the efficiency with values larger
than in the linear regime for an optimal choice of the chemical driving force.
We show that corresponding efficiencies can be of the order of 50%. A simple
analytic argument allows us to estimate the efficiency in this irreversible
regime for small external forces.Comment: 16 pages, 10 figure
Molecular motor that never steps backwards
We investigate the dynamics of a classical particle in a one-dimensional
two-wave potential composed of two periodic potentials, that are
time-independent and of the same amplitude and periodicity. One of the periodic
potentials is externally driven and performs a translational motion with
respect to the other. It is shown that if one of the potentials is of the
ratchet type, translation of the potential in a given direction leads to motion
of the particle in the same direction, whereas translation in the opposite
direction leaves the particle localized at its original location. Moreover,
even if the translation is random, but still has a finite velocity, an
efficient directed transport of the particle occurs.Comment: 4 pages, 5 figures, Phys. Rev. Lett. (in print
Self-organized Beating and Swimming of Internally Driven Filaments
We study a simple two-dimensional model for motion of an elastic filament
subject to internally generated stresses and show that wave-like propagating
shapes which can propel the filament can be induced by a self-organized
mechanism via a dynamic instability. The resulting patterns of motion do not
depend on the microscopic mechanism of the instability but only of the filament
rigidity and hydrodynamic friction. Our results suggest that simplified
systems, consisting only of molecular motors and filaments could be able to
show beating motion and self-propulsion.Comment: 8 pages, 2 figures, REVTe
Force Dependence of the Michaelis Constant in a Two-State Ratchet Model for Molecular Motors
We present a quantitative analysis of recent data on the kinetics of ATP
hydrolysis, which has presented a puzzle regarding the load dependence of the
Michaelis constant. Within the framework of coarse grained two-state ratchet
models, our analysis not only explains the puzzling data, but provides a
modified Michaelis law, which could be useful as a guide for future
experiments.Comment: 4 pages, 3 eps figures, accepted for publication on Physical Review
Letter
Renewal processes and fluctuation analysis of molecular motor stepping
We model the dynamics of a processive or rotary molecular motor using a
renewal processes, in line with the work initiated by Svoboda, Mitra and Block.
We apply a functional technique to compute different types of multiple-time
correlation functions of the renewal process, which have applications to
bead-assay experiments performed both with processive molecular motors, such as
myosin V and kinesin, and rotary motors, such as F1-ATPase
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