11,784 research outputs found
Dominance of extreme statistics in a prototype many-body Brownian ratchet
Many forms of cell motility rely on Brownian ratchet mechanisms that involve
multiple stochastic processes. We present a computational and theoretical study
of the nonequilibrium statistical dynamics of such a many-body ratchet, in the
specific form of a growing polymer gel that pushes a diffusing obstacle. We
find that oft-neglected correlations among constituent filaments impact
steady-state kinetics and significantly deplete the gel's density within
molecular distances of its leading edge. These behaviors are captured
quantitatively by a self-consistent theory for extreme fluctuations in
filaments' spatial distribution.Comment: 5 pages with 3 figures + 20 pages of Supplementary Material with 2
figures. Updated to agree with published version; published as a
Communication in J. Chem. Phy
Coexistence between fluid and crystalline phases of proteins in photosynthetic membranes
Photosystem II (PSII) and its associated light-harvesting complex II (LHCII)
are highly concentrated in the stacked grana regions of photosynthetic
thylakoid membranes. Within the membrane, PSII-LHCII supercomplexes can be
arranged in disordered packings, ordered arrays, or mixtures thereof. The
physical driving forces underlying array formation are unknown, complicating
attempts to determine a possible functional role for arrays in regulating light
harvesting or energy conversion efficiency. Here we introduce a coarse-grained
model of protein interactions in coupled photosynthetic membranes, focusing on
just two particle types that feature simple shapes and potential energies
motivated by structural studies. Reporting on computer simulations of the
model's equilibrium fluctuations, we demonstrate its success in reproducing
diverse structural features observed in experiments, including extended
PSII-LHCII arrays. Free energy calculations reveal that the appearance of
arrays marks a phase transition from the disordered fluid state to a
system-spanning crystal, which can easily be arrested by thermodynamic
constraints or slow dynamics. The region of fluid-crystal coexistence is broad,
encompassing much of the physiologically relevant parameter regime. Our results
suggest that grana membranes lie at or near phase coexistence, conferring
significant structural and functional flexibility to this densely packed
membrane protein system.Comment: 11 pages, 5 figure
Body-rock or lift-off in flow
Conditions are investigated under which a body lying at rest or rocking on a
solid horizontal surface can be removed from the surface by hydrodynamic forces
or instead continues rocking. The investigation is motivated by recent
observations on Martian dust movement as well as other small- and large-scale
applications. The nonlinear theory of fluid-body interaction here has unsteady
motion of an inviscid fluid interacting with a moving thin body. Various shapes
of body are addressed together with a range of initial conditions. The relevant
parameter space is found to be subtle as evolution and shape play substantial
roles coupled with scaled mass and gravity effects. Lift-off of the body from
the surface generally cannot occur without fluid flow but it can occur either
immediately or within a finite time once the fluid flow starts up: parameters
for this are found and comparisons are made with Martian observations.Comment: 28 pages, 9 figure
- …