3,283 research outputs found
Non-equilibrium raft-like membrane domains under continuous recycling
We present a model for the kinetics of spontaneous membrane domain (raft)
assembly that includes the effect of membrane recycling ubiquitous in living
cells. We show that the domains have a broad power-law distribution with an
average radius that scales with the 1/4 power of the domain lifetime when the
line tension at the domain edges is large. For biologically reasonable
recycling and diffusion rates the average domain radius is in the tens of nm
range, consistent with observations. This represents one possible link between
signaling (involving rafts) and traffic (recycling) in cells. Finally, we
present evidence that suggests that the average raft size may be the same for
all scale-free recycling schemes.Comment: 8 pages, 5 figure
Lateral phase separation in mixtures of lipids and cholesterol
In an effort to understand "rafts" in biological membranes, we propose phenomenological models for saturated and unsaturated lipid mixtures, and lipid-cholesterol mixtures. We consider simple couplings between the local composition and internal membrane structure, and their influence on transitions between liquid and gel membrane phases. Assuming that the gel transition temperature of the saturated lipid is shifted by the presence of the unsaturated lipid, and that cholesterol acts as an external field on the chain melting transition, a variety of phase diagrams are obtained. The phase diagrams for binary mixtures of saturated/unsaturated lipids and lipid/cholesterol are in semi-quantitative agreement with the experiments. Our results also apply to regions in the ternary phase diagram of lipid/lipid/cholesterol systems
Fission of a multiphase membrane tube
A common mechanism for intracellular transport is the use of controlled
deformations of the membrane to create spherical or tubular buds. While the
basic physical properties of homogeneous membranes are relatively well-known,
the effects of inhomogeneities within membranes are very much an active field
of study. Membrane domains enriched in certain lipids in particular are
attracting much attention, and in this Letter we investigate the effect of such
domains on the shape and fate of membrane tubes. Recent experiments have
demonstrated that forced lipid phase separation can trigger tube fission, and
we demonstrate how this can be understood purely from the difference in elastic
constants between the domains. Moreover, the proposed model predicts timescales
for fission that agree well with experimental findings
Stable patterns of membrane domains at corrugated substrates
Multi-component membranes such as ternary mixtures of lipids and cholesterol
can exhibit coexistence regions between two liquid phases. When such membranes
adhere to a corrugated substrate, the phase separation process strongly depends
on the interplay between substrate topography, bending rigidities, and line
tension of the membrane domains as we show theoretically via energy
minimization and Monte Carlo simulations. For sufficiently large bending
rigidity contrast between the two membrane phases, the corrugated substrate
truncates the phase separation process and leads to a stable pattern of
membrane domains. Our theory is consistent with recent experimental
observations and provides a possible control mechanism for domain patterns in
biological membranes.Comment: to appear in Physical Review Letter
Molecular motors robustly drive active gels to a critically connected state
Living systems often exhibit internal driving: active, molecular processes
drive nonequilibrium phenomena such as metabolism or migration. Active gels
constitute a fascinating class of internally driven matter, where molecular
motors exert localized stresses inside polymer networks. There is evidence that
network crosslinking is required to allow motors to induce macroscopic
contraction. Yet a quantitative understanding of how network connectivity
enables contraction is lacking. Here we show experimentally that myosin motors
contract crosslinked actin polymer networks to clusters with a scale-free size
distribution. This critical behavior occurs over an unexpectedly broad range of
crosslink concentrations. To understand this robustness, we develop a
quantitative model of contractile networks that takes into account network
restructuring: motors reduce connectivity by forcing crosslinks to unbind.
Paradoxically, to coordinate global contractions, motor activity should be low.
Otherwise, motors drive initially well-connected networks to a critical state
where ruptures form across the entire network.Comment: Main text: 21 pages, 5 figures. Supplementary Information: 13 pages,
8 figure
The effect of tidal flow directionality on tidal turbine performance characteristics
With many Tidal Energy Conversion (TEC) devices at full scale prototype stage there are two distinct design groups for Horizontal Axis Tidal Turbines (HATTs). Devices with a yaw mechanism allowing the turbine to always face into the flow, and devices with blades that can rotate through 180° to harness a strongly bi-directional flow. As marine turbine technology verges on the realm of economic viability this paper reveals the performance of Cardiff University's concept tidal turbine with its support structure either upstream or downstream and with various proximities between the rotating plane of the turbine and its support stanchion. Through the use of validated Computational Fluid Dynamics (CFD) modelling this work shows the optimal proximity between rotor plane and stanchion as well as establishing, in the given context, the use of a yaw mechanism to be superior to a bi-directional system from a performance perspective
An electrical analogy for the Pentland Firth tidal stream power resource
Several locations in the Pentland Firth, UK, have been earmarked for the deployment of separate farms of tidal turbines. However, recent numerical modelling suggests that these farms will be interdependent and that they must work together to optimize their collective performance. To explain this inter-dependence, in this paper we develop an electrical circuit analogy to describe flow through the Pentland Firth, in which parallel connections in the circuit represent different sub-channels formed by the islands of Swona, Stroma and the Pentland Skerries. The analogy is introduced in stages, beginning with turbines placed in a single channel, then turbines placed in a sub-channel connected in parallel to another sub-channel, and finally more complicated arrangements, in which turbines are installed both in parallel and in series within a multiply connected channel. The analogy leads to a general formula to predict the tidal power potential of turbines placed in a sub-channel connected in parallel to another sub-channel, and a predictive model for more complicated multiply connected channel arrangements. Power estimates made using the formula and predictive model (which may be applied using only measurements of the undisturbed natural tidal hydrodynamics) are shown to agree well with numerical model predictions for the Pentland Firth, providing useful insight into how to best develop the resource. © 2013 The Author(s) Published by the Royal Society. All rights reserved
Training and Onboarding initiatives in High Energy Physics experiments
In this paper we document the current analysis software training and
onboarding activities in several High Energy Physics (HEP) experiments: ATLAS,
CMS, LHCb, Belle II and DUNE. Fast and efficient onboarding of new
collaboration members is increasingly important for HEP experiments as analyses
and the related software become ever more complex with growing datasets. A
meeting series was held by the HEP Software Foundation (HSF) in 2022 for
experiments to showcase their initiatives. Here we document and analyse these
in an attempt to determine a set of key considerations for future experiments
Real-time intermembrane force measurements and imaging of lipid domain morphology during hemifusion
Membrane fusion is the core process in membrane trafficking and is essential for cellular transport of proteins and other biomacromolecules. During protein-mediated membrane fusion, membrane proteins are often excluded from the membrane-membrane contact, indicating that local structural transformations in lipid domains play a major role. However, the rearrangements of lipid domains during fusion have not been thoroughly examined. Here using a newly developed Fluorescence Surface Forces Apparatus (FL-SFA), migration of liquid-disordered clusters and depletion of liquid-ordered domains at the membrane-membrane contact are imaged in real time during hemifusion of model lipid membranes, together with simultaneous force-distance and lipid membrane thickness measurements. The load and contact time-dependent hemifusion results show that the domain rearrangements decrease the energy barrier to fusion, illustrating the significance of dynamic domain transformations in membrane fusion processes. Importantly, the FL-SFA can unambiguously correlate interaction forces and in situ imaging in many dynamic interfacial systems.open0
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