25 research outputs found
Fluctuations in Mass-Action Equilibrium of Protein Binding Networks
We consider two types of fluctuations in the mass-action equilibrium in
protein binding networks. The first type is driven by relatively slow changes
in total concentrations (copy numbers) of interacting proteins. The second
type, to which we refer to as spontaneous, is caused by quickly decaying
thermodynamic deviations away from the equilibrium of the system. As such they
are amenable to methods of equilibrium statistical mechanics used in our study.
We investigate the effects of network connectivity on these fluctuations and
compare them to their upper and lower bounds. The collective effects are shown
to sometimes lead to large power-law distributed amplification of spontaneous
fluctuations as compared to the expectation for isolated dimers. As a
consequence of this, the strength of both types of fluctuations is positively
correlated with the overall network connectivity of proteins forming the
complex. On the other hand, the relative amplitude of fluctuations is
negatively correlated with the abundance of the complex. Our general findings
are illustrated using a real network of protein-protein interactions in baker's
yeast with experimentally determined protein concentrations.Comment: 4 pages, 3 figure
Modeling microbial cross-feeding at intermediate scale portrays community dynamics and species coexistence
Social interaction between microbes can be described at many levels of
details, ranging from the biochemistry of cell-cell interactions to the
ecological dynamics of populations. Choosing the best level to model microbial
communities without losing generality remains a challenge. Here we propose to
model cross-feeding interactions at an intermediate level between genome-scale
metabolic models of individual species and consumer-resource models of
ecosystems, which is suitable to empirical data. We applied our method to three
published examples of multi-strain Escherichia coli communities with increasing
complexity consisting of uni-, bi-, and multi-directional cross-feeding of
either substitutable metabolic byproducts or essential nutrients. The
intermediate-scale model accurately described empirical data and could quantify
exchange rates elusive by other means, such as the byproduct secretions, even
for a complex community of 14 amino acid auxotrophs. We used the three models
to study each community's limits of robustness to perturbations such as
variations in resource supply, antibiotic treatments and invasion by other
"cheaters" species. Our analysis provides a foundation to quantify
cross-feeding interactions from experimental data, and highlights the
importance of metabolic exchanges in the dynamics and stability of microbial
communities.Comment: 6 figure
Expansion Around the Mean-Field Solution of the Bak-Sneppen Model
We study a recently proposed equation for the avalanche distribution in the
Bak-Sneppen model. We demonstrate that this equation indirectly relates
,the exponent for the power law distribution of avalanche sizes, to ,
the fractal dimension of an avalanche cluster.We compute this relation
numerically and approximate it analytically up to the second order of expansion
around the mean field exponents. Our results are consistent with Monte Carlo
simulations of Bak-Sneppen model in one and two dimensions.Comment: 5 pages, 2 ps-figures iclude
Critical exponents of the anisotropic Bak-Sneppen model
We analyze the behavior of spatially anisotropic Bak-Sneppen model. We
demonstrate that a nontrivial relation between critical exponents tau and
mu=d/D, recently derived for the isotropic Bak-Sneppen model, holds for its
anisotropic version as well. For one-dimensional anisotropic Bak-Sneppen model
we derive a novel exact equation for the distribution of avalanche spatial
sizes, and extract the value gamma=2 for one of the critical exponents of the
model. Other critical exponents are then determined from previously known
exponent relations. Our results are in excellent agreement with Monte Carlo
simulations of the model as well as with direct numerical integration of the
new equation.Comment: 8 pages, three figures included with psfig, some rewriting, + extra
figure and table of exponent
Avalanche Dynamics in Evolution, Growth, and Depinning Models
The dynamics of complex systems in nature often occurs in terms of
punctuations, or avalanches, rather than following a smooth, gradual path. A
comprehensive theory of avalanche dynamics in models of growth, interface
depinning, and evolution is presented. Specifically, we include the Bak-Sneppen
evolution model, the Sneppen interface depinning model, the Zaitsev flux creep
model, invasion percolation, and several other depinning models into a unified
treatment encompassing a large class of far from equilibrium processes. The
formation of fractal structures, the appearance of noise, diffusion with
anomalous Hurst exponents, Levy flights, and punctuated equilibria can all be
related to the same underlying avalanche dynamics. This dynamics can be
represented as a fractal in spatial plus one temporal dimension. We develop
a scaling theory that relates many of the critical exponents in this broad
category of extremal models, representing different universality classes, to
two basic exponents characterizing the fractal attractor. The exact equations
and the derived set of scaling relations are consistent with numerical
simulations of the above mentioned models.Comment: 27 pages in revtex, no figures included. Figures or hard copy of the
manuscript supplied on reques
The ALICE experiment at the CERN LHC
ALICE (A Large Ion Collider Experiment) is a general-purpose, heavy-ion detector at the CERN LHC which focuses on QCD, the strong-interaction sector of the Standard Model. It is designed to address the physics of strongly interacting matter and the quark-gluon plasma at extreme values of energy density and temperature in nucleus-nucleus collisions. Besides running with Pb ions, the physics programme includes collisions with lighter ions, lower energy running and dedicated proton-nucleus runs. ALICE will also take data with proton beams at the top LHC energy to collect reference data for the heavy-ion programme and to address several QCD topics for which ALICE is complementary to the other LHC detectors. The ALICE detector has been built by a collaboration including currently over 1000 physicists and engineers from 105 Institutes in 30 countries. Its overall dimensions are 161626 m3 with a total weight of approximately 10 000 t. The experiment consists of 18 different detector systems each with its own specific technology choice and design constraints, driven both by the physics requirements and the experimental conditions expected at LHC. The most stringent design constraint is to cope with the extreme particle multiplicity anticipated in central Pb-Pb collisions. The different subsystems were optimized to provide high-momentum resolution as well as excellent Particle Identification (PID) over a broad range in momentum, up to the highest multiplicities predicted for LHC. This will allow for comprehensive studies of hadrons, electrons, muons, and photons produced in the collision of heavy nuclei. Most detector systems are scheduled to be installed and ready for data taking by mid-2008 when the LHC is scheduled to start operation, with the exception of parts of the Photon Spectrometer (PHOS), Transition Radiation Detector (TRD) and Electro Magnetic Calorimeter (EMCal). These detectors will be completed for the high-luminosity ion run expected in 2010. This paper describes in detail the detector components as installed for the first data taking in the summer of 2008