852 research outputs found
On Shape Transformations and Shape Fluctuations of Cellular Compartments and Vesicles
We discuss the shape formation and shape transitions of simple bilayer vesicles in context with their role in biology. In the first part several classes of shape changes of vesicles of one lipid component are described and it is shown that these can be explained in terms of the bending energy concept in particular augmented by the bilayer coupling hypothesis. In the second
part shape changes and vesicle fission of vesicles composed of membranes of lipid mixtures are reported. These are explained in terms of coupling between local curvature and phase separation
Thermo-mechanic-electrical coupling in phospholipid monolayers near the critical point
Lipid monolayers have been shown to represent a powerful tool in studying
mechanical and thermodynamic properties of lipid membranes as well as their
interaction with proteins. Using Einstein's theory of fluctuations we here
demonstrate, that an experimentally derived linear relationship both between
transition entropy S and area A as well as between transition entropy and
charge q implies a linear relationships between compressibility \kappa_T, heat
capacity c_\pi, thermal expansion coefficient \alpha_T and electric capacity
CT. We demonstrate that these couplings have strong predictive power as they
allow calculating electrical and thermal properties from mechanical
measurements. The precision of the prediction increases as the critical point
TC is approached
Phase transformations in a model mesenchymal tissue
doi: 10.1088/1478-3967/1/2/006 http://iopscience.iop.org/1478-3975/1/2/006/Connective tissues, the most abundant tissue type of the mature mammalian body, consist of cells suspended in complex microenvironments known as extracellular matrices (ECMs). In the immature connective tissues (mesenchymes) encountered in developmental biology and tissue engineering applications, the ECMs contain varying amounts of randomly arranged fibers, and the physical state of the ECM changes as the fibers secreted by the cells undergo fibril and fiber assembly and organize into networks. In vitro composites consisting of assembling solutions of type I collagen, containing suspended polystyrene latex beads (~6 µm in diameter) with collagen-binding surface properties, provide a simplified model for certain physical aspects of developing mesenchymes. In particular, assembly-dependent topological (i.e., connectivity) transitions within the ECM could change a tissue from one in which cell-sized particles (e.g., latex beads or cells) are mechanically unlinked to one in which the particles are part of a mechanical continuum. Any particle-induced alterations in fiber organization would imply that cells could similarly establish physically distinct microdomains within tissues. Here we show that the presence of beads above a critical number density accelerates the sol-gel transition that takes place during the assembly of collagen into a globally interconnected network of fibers. The presence of this suprathreshold number of beads also dramatically changes the viscoelastic properties of the collagen matrix, but only when the initial concentration of soluble collagen is itself above a critical value. Our studies provide a starting point for the analysis of phase transformations of more complex biomaterials including developing and healing tissues as well as tissue substitutes containing living cells.This work was supported by the Deutsche Forschungsgemeinschaft (Sa. 246/22-4, a group grant SFB 266), the Fonds der Chemischen Industrie, and grants from the National Science Foundation to GF
(IBN-97100010) and SAN (IBN-9603838) and NASA to GF (NAG2-1611)
Diffusion of active tracers in fluctuating fields
The problem of a particle diffusion in a fluctuating scalar field is studied.
In contrast to most studies of advection diffusion in random fields we analyze
the case where the particle position is also coupled to the dynamics of the
field. Physical realizations of this problem are numerous and range from the
diffusion of proteins in fluctuating membranes and the diffusion of localized
magnetic fields in spin systems. We present exact results for the diffusion
constant of particles diffusing in dynamical Gaussian fields in the adiabatic
limit where the field evolution is much faster than the particle diffusion. In
addition we compute the diffusion constant perturbatively, in the weak coupling
limit where the interaction of the particle with the field is small, using a
Kubo-type relation. Finally we construct a simple toy model which can be solved
exactly.Comment: 13 pages, 1 figur
Phase ordering and shape deformation of two-phase membranes
Within a coupled-field Ginzburg-Landau model we study analytically phase
separation and accompanying shape deformation on a two-phase elastic membrane
in simple geometries such as cylinders, spheres and tori. Using an exact
periodic domain wall solution we solve for the shape and phase ordering field,
and estimate the degree of deformation of the membrane. The results are
pertinent to a preferential phase separation in regions of differing curvature
on a variety of vesicles.Comment: 4 pages, submitted to PR
Shape Changes of Self-Assembled Actin Bilayer Composite Membranes
We report the self-assembly of thin actin shells beneath the membranes of
giant vesicles. Ion-carrier mediated influx of Mg2+ induces actin
polymerization in the initially spherical vesicles. Buckling of the vesicles
and the formation of blisters after thermally induced bilayer expansion is
demonstrated. Bilayer flickering is dominated by tension generated by its
coupling to the actin cortex. Quantitative flicker analysis suggests the
bilayer and the actin cortex are separated by 0.4 \mum to 0.5 \mum due to
undulation forces.Comment: pdf-file, has been accepted by PR
Stability Design for Concrete Mattresses
ABSTRACT Concrete mattresses are widely used within the offshore industry for scour control, stabilisation and protection. There is currently no uniform industry methodology for concrete mattress stability design for any application of concrete mattresses. This paper reviews design works previously carried out for a project in the North West Shelf of Australia and proposes an industry design methodology. This methodology encompasses the application of recognised pipeline codes and standards, and the requirements and results of 2D and 3D Computational Fluid Dynamics (CFD) modelling as well as physical model testing
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
Mass-luminosity relation for FGK main sequence stars: metallicity and age contributions
The stellar mass-luminosity relation (MLR) is one of the most famous
empirical "laws", discovered in the beginning of the 20th century. MLR is still
used to estimate stellar masses for nearby stars, particularly for those that
are not binary systems, hence the mass cannot be derived directly from the
observations. It's well known that the MLR has a statistical dispersion which
cannot be explained exclusively due to the observational errors in luminosity
(or mass). It is an intrinsic dispersion caused by the differences in age and
chemical composition from star to star. In this work we discuss the impact of
age and metallicity on the MLR. Using the recent data on mass, luminosity,
metallicity, and age for 26 FGK stars (all members of binary systems, with
observational mass-errors <= 3%), including the Sun, we derive the MLR taking
into account, separately, mass-luminosity, mass-luminosity-metallicity, and
mass-luminosity-metallicity-age. Our results show that the inclusion of age and
metallicity in the MLR, for FGK stars, improves the individual mass estimation
by 5% to 15%.Comment: 7 pages, 4 figures, 1 table, accepted in Astrophysics and Space
Scienc
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