36 research outputs found
The kinesin walk: a dynamic model with elastically coupled heads
Recently individual two-headed kinesin molecules have been studied in in
vitro motility assays revealing a number of their peculiar transport
properties. In this paper we propose a simple and robust model for the kinesin
stepping process with elastically coupled Brownian heads showing all of these
properties. The analytic and numerical treatment of our model results in a very
good fit to the experimental data and practically has no free parameters.
Changing the values of the parameters in the restricted range allowed by the
related experimental estimates has almost no effect on the shape of the curves
and results mainly in a variation of the zero load velocity which can be
directly fitted to the measured data. In addition, the model is consistent with
the measured pathway of the kinesin ATPase.Comment: 6 pages, 3 figure
Collective motion of cells: from experiments to models
Swarming or collective motion of living entities is one of the most common
and spectacular manifestations of living systems having been extensively
studied in recent years. A number of general principles have been established.
The interactions at the level of cells are quite different from those among
individual animals therefore the study of collective motion of cells is likely
to reveal some specific important features which are overviewed in this paper.
In addition to presenting the most appealing results from the quickly growing
related literature we also deliver a critical discussion of the emerging
picture and summarize our present understanding of collective motion at the
cellular level. Collective motion of cells plays an essential role in a number
of experimental and real-life situations. In most cases the coordinated motion
is a helpful aspect of the given phenomenon and results in making a related
process more efficient (e.g., embryogenesis or wound healing), while in the
case of tumor cell invasion it appears to speed up the progression of the
disease. In these mechanisms cells both have to be motile and adhere to one
another, the adherence feature being the most specific to this sort of
collective behavior. One of the central aims of this review is both presenting
the related experimental observations and treating them in the light of a few
basic computational models so as to make an interpretation of the phenomena at
a quantitative level as well.Comment: 24 pages, 25 figures, 13 reference video link
A question of scale
If you search for 'collective behaviour' with your web browser most of the
texts popping up will be about group activities of humans, including riots,
fashion and mass panic. Nevertheless, collective behaviour is also considered
to be an important aspect of observed phenomena in atoms and molecules, for
example, during spontaneous magnetization. In your web search, you might also
find articles on collectively migrating bacteria, insects or birds; or
phenomena where groups of organisms or non- living objects synchronize their
signals or motion (think of fireflies flashing in unison or people clapping in
phase during rhythmic applause).Comment: Concepts essay, published in Nature
http://www.nature.com/nature/journal/v411/n6836/full/411421a0.htm
Overlapping modularity at the critical point of k-clique percolation
One of the most remarkable social phenomena is the formation of communities
in social networks corresponding to families, friendship circles, work teams,
etc. Since people usually belong to several different communities at the same
time, the induced overlaps result in an extremely complicated web of the
communities themselves. Thus, uncovering the intricate community structure of
social networks is a non-trivial task with great potential for practical
applications, gaining a notable interest in the recent years. The Clique
Percolation Method (CPM) is one of the earliest overlapping community finding
methods, which was already used in the analysis of several different social
networks. In this approach the communities correspond to k-clique percolation
clusters, and the general heuristic for setting the parameters of the method is
to tune the system just below the critical point of k-clique percolation.
However, this rule is based on simple physical principles and its validity was
never subject to quantitative analysis. Here we examine the quality of the
partitioning in the vicinity of the critical point using recently introduced
overlapping modularity measures. According to our results on real social- and
other networks, the overlapping modularities show a maximum close to the
critical point, justifying the original criteria for the optimal parameter
settings.Comment: 20 pages, 6 figure
Phase transition in the scalar noise model of collective motion in three dimensions
We consider disorder-order phase transitions in the three-dimensional version
of the scalar noise model (SNM) of flocking. Our results are analogous to those
found for the two-dimensional case. For small velocity (v <= 0.1) a continuous,
second-order phase transition is observable, with the diffusion of nearby
particles being isotropic. By increasing the particle velocities the phase
transition changes to first order, and the diffusion becomes anisotropic. The
first-order transition in the latter case is probably caused by the interplay
between anisotropic diffusion and periodic boundary conditions, leading to a
boundary condition dependent symmetry breaking of the solutions.Comment: 7 pages, 6 figures; submitted to EPJ on 17 of April, 200