42 research outputs found
Tubulation pattern of membrane vesicles coated with bio filaments
Narrow membrane tubes are commonly pulled out from the surface of
phospholipid vesicles using forces applied either through laser or magnetic
tweezers or through the action of processive motor proteins. Recent examples
have emerged where such tubes spontaneously grow from vesicles coated with
bioactive cytoskeletal filaments (e.g. FtsZ, microtubule) in the presence GTP.
We show how a soft vesicle deforms due to the interplay between its topology,
local curvature and the forces due to the active filaments. We present results
from Dynamically Triangulated Monte Carlo simulations of a spherical continuum
membrane coated with a nematic field and show how the intrinsic curvature of
the filaments and their ordering interactions drive membrane tubulation. We
predict interesting patterns of nematic defects, on curved 2D membrane
surfaces, which promote tube formation. Implication of our model for more
dynamic cases where vesicles coated with an active mixture of microtubule and
myosin show shape oscillation, are also discussed. All these cases point to a
common theme that defect locations on 2D membrane surfaces are hot spots of
membrane deformation activity.Comment: 8 pages, 7 figure
Chemosensing in microorganisms to practical biosensors
Microorganisms like bacteria can sense concentration of chemo-attractants in
its medium very accurately. They achieve this through interaction between the
receptors on their cell surface and the chemo-attractant molecules (like
sugar). But the physical processes like diffusion set some limits on the
accuracy of detection which was discussed by Berg and Purcell in the late
seventies. We have a re-look at their work in order to assess what insight it
may offer towards making efficient, practical biosensors. We model the
functioning of a typical biosensor as a reaction-diffusion process in a
confined geometry. Using available data first we characterize the system by
estimating the kinetic constants for the binding/unbinding reactions between
the chemo-attractants and the receptors. Then we compute the binding flux for
this system which Berg and Purcell had discussed. But unlike in microorganisms
where the interval between successive measurements determines the efficiency of
the nutrient searching process, it turns out that biosensors depend on long
time properties like signal saturation time which we study in detail. We also
develop a mean field description of the kinetics of the system.Comment: 6 pages, 7 figure
Multiscaling in Models of Magnetohydrodynamic Turbulence
From a direct numerical simulation of the MHD equations we show, for the
first time, that velocity and magnetic-field structure functions exhibit
multiscaling, extended self similarity (ESS), and generalized extended self
similarity (GESS). We also propose a new shell model for homogeneous and
isotropic MHD turbulence, which preserves all the invariants of ideal MHD,
reduces to a well-known shell model for fluid turbulence for zero magnetic
field, has no adjustable parameters apart from Reynolds numbers, and exhibits
the same multiscaling, ESS, and GESS as the MHD equations. We also study
dissipation-range asymptotics and the inertial- to dissipation-range crossover.Comment: 5 pages, REVTEX, 4 figures (eps
Stretching force dependent transitions in single stranded DNA
Mechanical properties of DNA, in particular their stretch dependent extension
and their loop formation characteristics, have been recognized as an effective
probe for understanding the possible biochemical role played by them in a
living cell. Single stranded DNA (ssDNA), which, till recently was presumed to
be an simple flexible polymer continues to spring surprises. Synthetic ssDNA,
like polydA (polydeoxyadenosines) has revealed an intriguing force-extension
(FX) behavior exhibiting two plateaus, absent in polydT (polydeoxythymidines)
for example. Loop closing time in polydA had also been found to scale
exponentially with inverse temperature, unexpected from generic models of
homopolymers. Here we present a new model for polydA which incorporates both a
helix-coil transition and a over-stretching transition, accounting for the two
plateaus. Using transfer matrix calculation and Monte-Carlo simulation we show
that the model reproduces different sets of experimental observations,
quantitatively. It also predicts interesting reentrant behavior in the
temperature-extension characteristics of polydA, which is yet to be verified
experimentally.Comment: 5 pages, 3 figure
Predicting the coherence resonance curve using a semi-analytical treatment
Emergence of noise induced regularity or Coherence Resonance in nonlinear
excitable systems is well known. We explain theoretically why the normalized
variance () of inter spike time intervals, which is a measure of
regularity in such systems, has a unimodal profile. Our semi-analytic treatment
of the associated spiking process produces a general yet simple formula for
, which we show is in very good agreement with numerics in two test
cases, namely the FitzHugh-Nagumo model and the Chemical Oscillator model.Comment: 5 pages, 5 figure