1,841 research outputs found
Control of Spatially Heterogeneous and Time-Varying Cellular Reaction Networks: A New Summation Law
A hallmark of a plethora of intracellular signaling pathways is the spatial
separation of activation and deactivation processes that potentially results in
precipitous gradients of activated proteins. The classical Metabolic Control
Analysis (MCA), which quantifies the influence of an individual process on a
system variable as the control coefficient, cannot be applied to spatially
separated protein networks. The present paper unravels the principles that
govern the control over the fluxes and intermediate concentrations in spatially
heterogeneous reaction networks. Our main results are two types of the control
summation theorems. The first type is a non-trivial generalization of the
classical theorems to systems with spatially and temporally varying
concentrations. In this generalization, the process of diffusion, which enters
as the result of spatial concentration gradients, plays a role similar to other
processes such as chemical reactions and membrane transport. The second
summation theorem is completely novel. It states that the control by the
membrane transport, the diffusion control coefficient multiplied by two, and a
newly introduced control coefficient associated with changes in the spatial
size of a system (e.g., cell), all add up to one and zero for the control over
flux and concentration. Using a simple example of a kinase/phosphatase system
in a spherical cell, we speculate that unless active mechanisms of
intracellular transport are involved, the threshold cell size is limited by the
diffusion control, when it is beginning to exceed the spatial control
coefficient significantly.Comment: 19 pages, AMS-LaTeX, 6 eps figures included with geompsfi.st
Conformational transformations induced by the charge-curvature interaction at finite temperature
The role of thermal fluctuations on the conformational dynamics of a single
closed filament is studied. It is shown that, due to the interaction between
charges and bending degrees of freedom, initially circular aggregates may
undergo transformation to polygonal shape. The transition occurs both in the
case of hardening and softening charge-bending interaction. In the former case
the charge and curvature are smoothly distributed along the chain while in the
latter spontaneous kink formation is initiated. The transition to a
non-circular conformation is analogous to the phase transition of the second
kind.Comment: 23 pages (Latex), 10 figures (Postscript), 2 biblio file (bib-file
and bbl-file
Veneziano Amplitudes, Spin Chains and String Models
In a series of recently published papers we reanalyzed the existing
treatments of Veneziano and Veneziano-like amplitudes and the models associated
with these amplitudes. In this work we demonstrate that the already obtained
new partition function for these amplitudes can be exactly mapped into that for
the Polychronakos-Frahm (P-F) spin chain model. This observation allows us to
recover many of the existing string-theoretic models, including the most recent
ones.Comment: 38 page
Measurement of the Thermal Expansion Coefficient of Quasi Mono-Crystal Pyrolitic Graphite Samples
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