1,259 research outputs found
Serially-regulated biological networks fully realize a constrained set of functions
We show that biological networks with serial regulation (each node regulated
by at most one other node) are constrained to {\it direct functionality}, in
which the sign of the effect of an environmental input on a target species
depends only on the direct path from the input to the target, even when there
is a feedback loop allowing for multiple interaction pathways. Using a
stochastic model for a set of small transcriptional regulatory networks that
have been studied experimentally, we further find that all networks can achieve
all functions permitted by this constraint under reasonable settings of
biochemical parameters. This underscores the functional versatility of the
networks.Comment: 9 pages, 3 figure
Dynamic Fluctuation Phenomena in Double Membrane Films
Dynamics of double membrane films is investigated in the long-wavelength
limit including the overdamped squeezing mode. We demonstrate that thermal
fluctuations essentially modify the character of the mode due to its nonlinear
coupling to the transversal shear hydrodynamic mode. The corresponding Green
function acquires as a function of the frequency a cut along the imaginary
semi-axis. Fluctuations lead to increasing the attenuation of the squeezing
mode it becomes larger than the `bare' value.Comment: 7 pages, Revte
Coiling Instabilities in Multilamellar Tubes
Myelin figures are densely packed stacks of coaxial cylindrical bilayers that
are unstable to the formation of coils or double helices. These myelin figures
appear to have no intrinsic chirality. We show that such cylindrical membrane
stacks can develop an instability when they acquire a spontaneous curvature or
when the equilibrium distance between membranes is decreased. This instability
breaks the chiral symmetry of the stack and may result in coiling. A
unilamellar cylindrical vesicle, on the other hand, will develop an
axisymmetric instability, possibly related to the pearling instability.Comment: 6 pages, 2 figure
Instability of Myelin Tubes under Dehydration: deswelling of layered cylindrical structures
We report experimental observations of an undulational instability of myelin
figures. Motivated by this, we examine theoretically the deformation and
possible instability of concentric, cylindrical, multi-lamellar membrane
structures. Under conditions of osmotic stress (swelling or dehydration), we
find a stable, deformed state in which the layer deformation is given by \delta
R ~ r^{\sqrt{B_A/(hB)}}, where B_A is the area compression modulus, B is the
inter-layer compression modulus, and h is the repeat distance of layers. Also,
above a finite threshold of dehydration (or osmotic stress), we find that the
system becomes unstable to undulations, first with a characteristic wavelength
of order \sqrt{xi d_0}, where xi is the standard smectic penetration depth and
d_0 is the thickness of dehydrated region.Comment: 5 pages + 3 figures [revtex 4
Tropical tele-connections to the Mediterranean climate and weather
Some strong natural fluctuations of climate in the Eastern Mediterranean (EM) region are shown to be connected to the major tropical systems. Potential relations between EM rainfall extremes to tropical systems, e.g. El Niño, Indian Monsoon and hurricanes, are demonstrated. For a specific event, high resolution modelling of the severe flood on 3-5 December 2001 in Israel suggests a relation to hurricane Olga. In order to understand the factors governing the EM climate variability in the summer season, the relationship between extreme summer temperatures and the Indian Monsoon was examined. Other tropical factors like the Red-Sea Trough system and the Saharan dust are also likely to contribute to the EM climate variability
Quantum Manipulations of Small Josephson Junctions
Low-capacitance Josephson junction arrays in the parameter range where single
charges can be controlled are suggested as possible physical realizations of
the elements which have been considered in the context of quantum computers. We
discuss single and multiple quantum bit systems. The systems are controlled by
applied gate voltages, which also allow the necessary manipulation of the
quantum states. We estimate that the phase coherence time is sufficiently long
for experimental demonstration of the principles of quantum computation.Comment: RevTex, 15 pages,4 postscript figures, uuencoded, submitted to Phys.
Rev. Lett., estimates of the experimental parameters correcte
Limited biomass recovery from gold mining in Amazonian forests
1. Gold mining has rapidly increased across the Amazon Basin in recent years, especially in the Guiana shield, where it is responsible for >90% of total deforestation. However, the ability of forests to recover from gold mining activities remains largely unquantified.
2. Forest inventory plots were installed on recently abandoned mines in two major mining regions in Guyana, and re‐censused 18 months later, to provide the first ground‐based quantification of gold mining impacts on Amazon forest biomass recovery.
3. We found that woody biomass recovery rates on abandoned mining pits and tailing ponds are among the lowest ever recorded for tropical forests, with close to no woody biomass recovery after 3–4 years.
4. On the overburden sites (i.e. areas not mined but where excavated soil is deposited), however, above‐ground biomass recovery rates (0.4–5.4 Mg ha−1 year−1) were within the range of those recorded in other secondary forests across the Neotropics following abandonment of pastures and agricultural lands.
5. Our results suggest that forest recovery is more strongly limited by severe mining‐induced depletion of soil nutrients, especially nitrogen, than by mercury contamination, due to slowing of growth in nutrient‐stripped soils.
6. We estimate that the slow recovery rates in mining pits and ponds currently reduce carbon sequestration across Amazonian secondary forests by ~21,000 t C/year, compared to the carbon that would have accumulated following more traditional land uses such as agriculture or pasture.
7. Synthesis and applications. To achieve large‐scale restoration targets, Guyana and other Amazonian countries will be challenged to remediate previously mined lands. The recovery process is highly dependent on nitrogen availability rather than mercury contamination, affecting woody biomass regrowth. The significant recovery in overburden zones indicates that one potential active remediation strategy to promote biomass recovery may be to backfill mining pits and ponds with excavated soil
Entropy and Quantum Kolmogorov Complexity: A Quantum Brudno's Theorem
In classical information theory, entropy rate and Kolmogorov complexity per
symbol are related by a theorem of Brudno. In this paper, we prove a quantum
version of this theorem, connecting the von Neumann entropy rate and two
notions of quantum Kolmogorov complexity, both based on the shortest qubit
descriptions of qubit strings that, run by a universal quantum Turing machine,
reproduce them as outputs.Comment: 26 pages, no figures. Reference to publication added: published in
the Communications in Mathematical Physics
(http://www.springerlink.com/content/1432-0916/
Orientation and loading condition dependence of fracture toughness in cortical bone
The fracture toughness at crack initiation were determined for bovine cortical bone under tension (mode I), shear (mode II), and tear (mode III). A total of 140 compact tension specimens, compact shear specimens and triple pantleg (TP) specimens were used to measure fracture toughness under tension, shear, and tear, respectively. Multiple-sample compliance method was utilized to measure the critical strain energy release rate (G(c)) at the a/W= 0.55 (crack length, a, to specimen width, W, ratio). The critical stress intensity factor (K(c)) was also calculates from the critical loading (P(c)) of the specimens at the a/W= 0.55. The effect of the anisotropy of bone on its resistance to crack initiation under shear and tear loading was investigated as well. Fracture toughness of bone with precrack orientations parallel (designed as longitudinal fracture) and Vertical (designed as transverse fracture) to the longitudinal axis of bone were compared. In longitudinal fracture, the critical strain energy release rate (G(c)) of cortical bone under tension, shear, and tear was 644 +/- 102, 2430 +/- 836, and 1723 +/- 486 N/m, respectively. In transverse fracture, the critical strain energy release rate (G(c)) of cortical bone under tension, shear, and tear was 1374 +/- 183, 4710 +/- 1284, and 4016 +/- 948 N/m, respectively. An unpaired t-test analysis demonstrated that the crack initiation fracture toughness of bone under shear and tear loading were significantly greater than that under tensile loading in both longitudinal and transverse fracture (P < 0.0001 for all). Our results also suggest that cortical bone has been ''designed'' to prevent crack initiation in transverse fracture under tension, shear, and tear. (C) 2000 Elsevier Science S.A. All rights reserved
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