Plenarvorträge - DPG-Tagungen

Arbeitskreis Biologische Physik Freitag
AKB 50.41 Fr 10:30 B
Overdamped Buckling Dynamics — •Oskar Hallatschek 1 , Erwin
Frey 1,2 , and Klaus Kroy 1 — 1 Abteilung Theorie, Hahn-Meitner
Institut, Glienicker Str. 100, 14109 Berlin, Germany — 2 Fachbereich
Physik, Freie Universität,14195 Berlin, Germany
We discuss the generic relaxation dynamics of a weakly bending incompressible
rod in a viscous environment. We show that the decay of the
initially imposed transverse undulations creates two expanding boundary
layers that grow at the cost of a longitudinally inert bulk. In the bulk,
longitudinal expansion is completely suppressed by the viscous friction
on the boundary regions. We have calculated tension profiles based on
coarse grained equations of motion for the position dependent tension,
from which all other relevant observables can be calculated. We find that
the different scenarios of tension release can conveniently be classified
in terms of a roughness exponent β as a measure for the type of initial
excitation.
AKB 50.42 Fr 10:30 B
Linear response of a grafted semiflexible polymer to a uniform
field — •Panayotis Benetatos 1 and Erwin Frey 1,2 — 1 Hahn-
Meitner-Institut, Abteilung Theorie, Berlin — 2 Fachbereich Physik, Freie
Universitaet, Berlin
We study the linear response of a semiflexible polymer to a uniform
force field. We use the worm-like chain model and we consider a polymer
which has one end grafted and the other free. In contrast to a force
applied at the free end, the field introduces a nonlocal interaction along
the polymer contour. Because of this nonlocality, using the free chain
propagator (two-point orientational probability density) for the calculation
of the linear response of the polymer extension in the direction of
the field fails. We can apply the propagator method, however, in order to
calculate the linear response of the end-point tangent vector orientation
where locality is restored. We then use this result and make an Ansatz
to obtain the linear response of the extension. Our analytic results are
functions of the total contour length, the persistence length, and the orientation
of the field. We discuss how the response to a field differs from
that to a force.
AKB 50.43 Fr 10:30 B
Depinning of weakly bending semiflexible polymers —
•Panayotis Benetatos 1 and Erwin Frey 1,2 — 1 Hahn-Meitner-
Institut, Abteilung Theorie, Berlin — 2 Fachbereich Physik, Freie
Universitaet, Berlin
We present a simple theoretical model for the thermal depinning of a
weakly bending semiflexible polymer in 1+1 dimensions. We consider a
discrete sequence of pinning sites along the polymer contour. Each pinning
site is characterized by a binding energy, a transverse width, and
an angular width. We use the necklace model of phase transitions in
quasi-one-dimensional systems and we obtain a phase transition in the
thermodynamic limit where the density of pinning sites goes to infinity
and the widths of a pinning site go to zero. This novel way to take
the thermodynamic limit is consistent with the weakly bending approximation
which is valid when the total contour length of the polymer is
smaller than its persistence length. We also consider the depinning in
1+d dimensions.
AKB 50.44 Fr 10:30 B
Random walks of molecular motors — •Stefan Klumpp 1 ,
Theo M. Nieuwenhuizen 1,2 , and Reinhard Lipowsky 1 —
1 Max-Planck-Institut für Kolloid- und Grenzflächenforschung, 14424
Potsdam — 2 Instituut voor Theoretische Fysica, Valckenierstraat 65,
1018 XE Amsterdam
Processive molecular motors perform directed walks along cytoskeletal
filaments. Upon unbinding from the filaments, they undergo non-directed
Brownian motion until they rebind to the same or another filament. The
random walks that result from the interplay of bound and unbound motors
movements are studied theoretically. These random walks exhibit
anomalous drift behaviour in open compartments and stationary concentration
profiles characterized by a balance of bound and unbound motor
currents in closed compartments [1]. Mutual exclusion of motors from
the binding sites of the filaments leads to traffic jams and a maximal
motor current at an intermediate motor concentration. These transport
phenomena mimic transport phenomena in cells and are accessible to in
vitro experiments.
[1] R. Lipowsky, S. Klumpp, and T. M. Nieuwenhuizen,
Phys. Rev. Lett. 87, 108101 (2001).
AKB 50.45 Fr 10:30 B
Gating charge effects for nerve excitation — •Gerhard Schmid,
Igor Goychuk, and Peter Hänggi — Institut für Physik, Universität
Augsburg
Voltage dependent ion channels mainly determine the electric properties
of axonal cell membranes. The ion channels thereby do not only allow
passage of ions trough the cell membrane, but they also contribute to the
membrane capacity, since the switching of the channel gates between an
open and a closed configuration is always connected with movement of
charge within the cell membrane. Especially, in case of moderately large
densities of ion channels, which are found, for example, in the nodes of
Ranvier, this may play a crucial role for nerve excitation. Our model system
is a modified Hodgkin-Huxley model which takes into account the
change in the membrane capacity. Additional terms, which are proportional
to the number of ion channels resp. the number of gates, extend the
original Hodgkin-Huxley model and allow the consideration of a – due
to the gating charges – time-dependent membrane capacity. Surprisingly,
the gating charge do not dramatically change the excitation behavior
even for extreme dense ion channel assemblies, instead, the membrane
capacity at rest exhibits a bell-shaped dependence on the ion channel
density. This work is supported by DFG (SFB 486).
AKB 50.46 Fr 10:30 B
Membrane mechanics and polymer decoration — •Thorsten
Auth and Gerhard Gompper — Institut für Festkörperforschung,
Forschungszentrum Jülich, 52425 Jülich
Polymers embedded into or attached to lipid bilayer membranes modify
the membrane’s mechanical properties. A prominent example of a
polymer-membrane system is the red blood cell [1]. The lipid bilayer
membrane can be modeled by a mathematical surface and a set of material
constants: bending rigidity, saddle-splay modulus, spontaneous curvature.
In many cases the effect of attached, adsorbed or free polymers
can be described by effective curvature elastic constants (see e. g. [2]).
The excluded-volume interaction is well known to strongly affect the
scaling behaviour of polymer chains. We have therefore developed a
Monte Carlo simulation method to investigate the effects of self-avoidance
and different polymer architectures on the curvature elastic constants for
low polymer coverage [3]. The method will be introduced, results for the
effective curvature constants will be presented and discussed together
with implications on the behaviour of membrane systems.
[1] N. Gov, A. G. Zilman and S. Safran; Phys. Rev. Lett. 90, 228101
(2003)
[2] C. Hiergeist and R. Lipowsky; J. Phys. II (France) 6, 1465 (1996)
[3] T. Auth and G. Gompper; Phys. Rev. E 68, 051801 (2003)
AKB 50.47 Fr 10:30 B
Elastic and caloric properties of model membranes at the chain
melting transition — •Wolfgang Manglkammer and Jan K.
Krüger — Universität des Saarlandes, Experimentalphysik, Postfach
151150, D-66041 Saarbrücken, GERMANY
One remarkable feature of lipid membranes, which often serve as a
model for ’real’ biological membranes, is the so-called main transition,
but its nature is not yet clear. When the hydrocarbon chains are melting,
the lipid bilayer transforms from the gel phase into the fluid Lα phase.
We investigated the hypersonic properties of oriented, solid supported
Phosphatidylcholine bilayer stacks of different chain length by Brillouin
spectroscopy as a function of hydration and temperature. The elastic
moduli show huge steps which accompany several phase transitions.
Therefore the transition points can easily be defined, what allows us
to draw a temperature-composition phase diagram.
The elastic behaviour near the main transition is related to the specific
heat as obtained from multi-lamellar vesicles by quasi-isothermal Temperature
Modulated DSC. The heat capacity shows critical behaviour on
both sides of the transition. Such pre-critical features were not found
for the high frequency elastic moduli, although they were reported in
literature for low frequency elastic experiments.
AKB 50.48 Fr 10:30 B
Vesicle adhesion and cohesion at finite temperature — •Thomas
Gruhn and Reinhard Lipowsky — MPI for Colloids and Interfaces
14424 Potsdam
Biological cells are the fundamental building blocks of living organisms.
They are surrounded by a closed membrane surface that serves as
an interface to the outside world. In many cases cells act very similar