35 research outputs found
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
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
Stuttering Min oscillations within E. coli bacteria: A stochastic polymerization model
We have developed a 3D off-lattice stochastic polymerization model to study
subcellular oscillation of Min proteins in the bacteria Escherichia coli, and
used it to investigate the experimental phenomenon of Min oscillation
stuttering. Stuttering was affected by the rate of immediate rebinding of MinE
released from depolymerizing filament tips (processivity), protection of
depolymerizing filament tips from MinD binding, and fragmentation of MinD
filaments due to MinE. Each of processivity, protection, and fragmentation
reduces stuttering, speeds oscillations, and reduces MinD filament lengths.
Neither processivity or tip-protection were, on their own, sufficient to
produce fast stutter-free oscillations. While filament fragmentation could, on
its own, lead to fast oscillations with infrequent stuttering; high levels of
fragmentation degraded oscillations. The infrequent stuttering observed in
standard Min oscillations are consistent with short filaments of MinD, while we
expect that mutants that exhibit higher stuttering frequencies will exhibit
longer MinD filaments. Increased stuttering rate may be a useful diagnostic to
find observable MinD polymerization in experimental conditions.Comment: 21 pages, 7 figures, missing unit for k_f inserte
Inertial- and Dissipation-Range Asymptotics in Fluid Turbulence
We propose and verify a wave-vector-space version of generalized extended
self similarity and broaden its applicability to uncover intriguing, universal
scaling in the far dissipation range by computing high-order (\leq 20\/)
structure functions numerically for: (1) the three-dimensional, incompressible
Navier Stokes equation (with and without hyperviscosity); and (2) the GOY shell
model for turbulence. Also, in case (2), with Taylor-microscale Reynolds
numbers 4 \times 10^{4} \leq Re_{\lambda} \leq 3 \times 10^{6}\/, we find
that the inertial-range exponents (\zeta_{p}\/) of the order - p\/
structure functions do not approach their Kolmogorov value p/3\/ as
Re_{\lambda}\/ increases.Comment: RevTeX file, with six postscript figures. epsf.tex macro is used for
figure insertion. Packaged using the 'uufiles' utilit