652 research outputs found
Molecular mechanistic origin of the toughness of natural adhesives, fibres and composites
Natural materials are renowned for their strength and toughness(1-5). Spider dragline silk has a breakage energy per unit weight two orders of magnitude greater than high tensile steel(1,6), and is representative of many other strong natural fibres(3,7,8). The abalone shell, a composite of calcium carbonate plates sandwiched between organic material, is 3,000 times more fracture resistant than a single crystal of the pure mineral(4,5). The organic component, comprising just a few per cent of the composite by weight(9), is thought to hold the key to nacre's fracture toughness(10,11). Ceramics laminated with organic material are more fracture resistant than non-laminated ceramics(11,12), but synthetic materials made of interlocking ceramic tablets bound by a few weight per cent of ordinary adhesives do not have a toughness comparable to nacre(13). We believe that the key to nacre's fracture resistance resides in the polymer adhesive, and here we reveal the properties of this adhesive by using the atomic force microscope(14) to stretch the organic molecules exposed on the surface of freshly cleaved nacre. The adhesive fibres elongate in a stepwise manner as folded domains or loops are pulled open. The elongation events occur for forces of a few hundred piconewtons, which are smaller than the forces of over a nanonewton required to break the polymer backbone in the threads. We suggest that this 'modular' elongation mechanism might prove to be quite general for conveying toughness to natural fibres and adhesives, and we predict that it might be found also in dragline silk
Thermodynamics and structure of self-assembled networks
We study a generic model of self-assembling chains which can branch and form
networks with branching points (junctions) of arbitrary functionality. The
physical realizations include physical gels, wormlike micells, dipolar fluids
and microemulsions. The model maps the partition function of a solution of
branched, self-assembling, mutually avoiding clusters onto that of a Heisenberg
magnet in the mathematical limit of zero spin components. The model is solved
in the mean field approximation. It is found that despite the absence of any
specific interaction between the chains, the entropy of the junctions induces
an effective attraction between the monomers, which in the case of three-fold
junctions leads to a first order reentrant phase separation between a dilute
phase consisting mainly of single chains, and a dense network, or two network
phases. Independent of the phase separation, we predict the percolation
(connectivity) transition at which an infinite network is formed that partially
overlaps with the first-order transition. The percolation transition is a
continuous, non thermodynamic transition that describes a change in the
topology of the system. Our treatment which predicts both the thermodynamic
phase equilibria as well as the spatial correlations in the system allows us to
treat both the phase separation and the percolation threshold within the same
framework. The density-density correlation correlation has a usual
Ornstein-Zernicke form at low monomer densities. At higher densities, a peak
emerges in the structure factor, signifying an onset of medium-range order in
the system. Implications of the results for different physical systems are
discussed.Comment: Submitted to Phys. Rev.
Enlargement of ribbons in zebrafish hair cells increases calcium currents, but disrupts afferent spontaneous activity and timing of stimulus onset.
In sensory hair cells of auditory and vestibular organs, the ribbon synapse is required for the precise encoding of a wide range of complex stimuli. Hair cells have a unique presynaptic structure-the synaptic ribbon-that organizes both synaptic vesicles and calcium channels at the active zone. Previous work has shown that hair-cell ribbon size is correlated with differences in postsynaptic activity. However, additional variability in postsynapse size presents a challenge to determining the specific role of ribbon size in sensory encoding. To selectively assess the impact of ribbon size on synapse function, we examined hair cells in transgenic zebrafish that have enlarged ribbons, without postsynaptic alterations. Morphologically, we found that enlarged ribbons had more associated vesicles and reduced presynaptic calcium-channel clustering. Functionally, hair cells with enlarged ribbons had larger global and ribbon-localized calcium currents. Afferent neuron recordings revealed that hair cells with enlarged ribbons resulted in reduced spontaneous spike rates. Additionally, despite larger presynaptic calcium signals, we observed fewer evoked spikes with longer latencies from stimulus onset. Taken together, our work indicates that hair-cell ribbon size influences the spontaneous spiking and the precise encoding of stimulus onset in afferent neurons.SIGNIFICANCE STATEMENTNumerous studies support that hair-cell ribbon size corresponds with functional sensitivity differences in afferent neurons and, in the case of inner hair cells of the cochlea, vulnerability to damage from noise trauma. Yet it is unclear whether ribbon size directly influences sensory encoding. Our study reveals that ribbon enlargement results in increased ribbon-localized calcium signals, yet reduces afferent spontaneous activity and disrupts the timing of stimulus onset-a distinct aspect of auditory and vestibular encoding. These observations suggest that varying ribbon size alone can influence sensory encoding, and give further insight into how hair cells transduce signals that cover a wide dynamic range of stimuli
Ising Universality in Three Dimensions: A Monte Carlo Study
We investigate three Ising models on the simple cubic lattice by means of
Monte Carlo methods and finite-size scaling. These models are the spin-1/2
Ising model with nearest-neighbor interactions, a spin-1/2 model with
nearest-neighbor and third-neighbor interactions, and a spin-1 model with
nearest-neighbor interactions. The results are in accurate agreement with the
hypothesis of universality. Analysis of the finite-size scaling behavior
reveals corrections beyond those caused by the leading irrelevant scaling
field. We find that the correction-to-scaling amplitudes are strongly dependent
on the introduction of further-neighbor interactions or a third spin state. In
a spin-1 Ising model, these corrections appear to be very small. This is very
helpful for the determination of the universal constants of the Ising model.
The renormalization exponents of the Ising model are determined as y_t = 1.587
(2), y_h = 2.4815 (15) and y_i = -0.82 (6). The universal ratio Q =
^2/ is equal to 0.6233 (4) for periodic systems with cubic symmetry.
The critical point of the nearest-neighbor spin-1/2 model is K_c=0.2216546
(10).Comment: 25 pages, uuencoded compressed PostScript file (to appear in Journal
of Physics A
Easier sieving through narrower pores: fluctuations and barrier crossing in flow-driven polymer translocation
We show that the injection of polymer chains into nanochannels becomes easier
as the channel becomes narrower. This counter intuitive result arises because
of a decrease in the diffusive time scale of the chains with increasing
confinement. The results are obtained by extending the de Gennes blob model of
confined polymers, and confirmed by hybrid molecular dynamics -
lattice-Boltzmann simulations.Comment: 5 pages, 3 figure
Magnesium pyridoxal-5′-phosphate glutamate, "A vitamin B 6 derivative", does not affect lipoprotein levels in patients with familial hypercholesterolaemia
Single-molecule experiments in biological physics: methods and applications
I review single-molecule experiments (SME) in biological physics. Recent
technological developments have provided the tools to design and build
scientific instruments of high enough sensitivity and precision to manipulate
and visualize individual molecules and measure microscopic forces. Using SME it
is possible to: manipulate molecules one at a time and measure distributions
describing molecular properties; characterize the kinetics of biomolecular
reactions and; detect molecular intermediates. SME provide the additional
information about thermodynamics and kinetics of biomolecular processes. This
complements information obtained in traditional bulk assays. In SME it is also
possible to measure small energies and detect large Brownian deviations in
biomolecular reactions, thereby offering new methods and systems to scrutinize
the basic foundations of statistical mechanics. This review is written at a
very introductory level emphasizing the importance of SME to scientists
interested in knowing the common playground of ideas and the interdisciplinary
topics accessible by these techniques. The review discusses SME from an
experimental perspective, first exposing the most common experimental
methodologies and later presenting various molecular systems where such
techniques have been applied. I briefly discuss experimental techniques such as
atomic-force microscopy (AFM), laser optical tweezers (LOT), magnetic tweezers
(MT), biomembrane force probe (BFP) and single-molecule fluorescence (SMF). I
then present several applications of SME to the study of nucleic acids (DNA,
RNA and DNA condensation), proteins (protein-protein interactions, protein
folding and molecular motors). Finally, I discuss applications of SME to the
study of the nonequilibrium thermodynamics of small systems and the
experimental verification of fluctuation theorems. I conclude with a discussion
of open questions and future perspectives.Comment: Latex, 60 pages, 12 figures, Topical Review for J. Phys. C (Cond.
Matt
Artificial immune systems
The biological immune system is a robust, complex, adaptive system that defends the body from foreign pathogens. It is able to categorize all cells (or molecules) within the body as self or nonself substances. It does this with the help of a distributed task force that has the intelligence to take action from a local and also a global perspective using its network of chemical messengers for communication. There are two major branches of the immune system. The innate immune system is an unchanging mechanism that detects and destroys certain invading organisms, whilst the adaptive immune system responds to previously unknown foreign cells and builds a response to them that can remain in the body over a long period of time. This remarkable information processing biological system has caught the attention of computer science in recent years
Critical report of current fisheries management measures implemented for the North Sea mixed demersal fisheries
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