27 research outputs found
The Different Effect of Electron-Electron Interaction on the Spectrum of Atoms and Quantum Dots
The electron-electron scattering rate of single particle excitations in atoms
is estimated and compared with the corresponding rate in quantum dots. It is
found that in alkali atoms single particle excitations do not acquire a width
due to electron-electron interaction, while in complex atoms they may. This
width is typically smaller than the single particle level spacing, and hence
does not affect the number of discrete single particle excitations resolved
below the ionization threshold. This situation is contrasted with that of
quantum dots where electron-electron interaction severely limits the number of
resolved excitations. Unlike the case of quantum dots, the scattering rate in
atoms is found to decrease with increasing excitation energy. The different
effect of electron-electron interaction on the spectrum of quantum dots and
atoms is traced to the different confining potentials in the two systems.Comment: 12 pages including 2 eps figure
Scaling behavior in steady-state contractile actomyosin network flow
Contractile actomyosin network flows are crucial for many cellular processes
including cell division and motility, morphogenesis and transport. How local
remodeling of actin architecture tunes stress production and dissipation and
regulates large-scale network flow remains poorly understood. Here, we generate
contractile actomyosin networks with rapid turnover in vitro, by encapsulating
cytoplasmic Xenopus egg extracts into cell-sized 'water-in-oil' droplets.
Within minutes, the networks reach a dynamic steady-state with continuous
inward flow. The networks exhibit homogenous, density-independent contraction
for a wide range of physiological conditions, indicating that the
myosin-generated stress driving contraction is proportional to the effective
network viscosity. We further find that the contraction rate approximately
scales with the network turnover rate, but this relation breaks down in the
presence of excessive crosslinking or branching. Our findings suggest that
cells use diverse biochemical mechanisms to generate robust, yet tunable, actin
flows by regulating two parameters: turnover rate and network geometry
Front-to-Rear Membrane Tension Gradient in Rapidly Moving Cells
AbstractMembrane tension is becoming recognized as an important mechanical regulator of motile cell behavior. Although membrane-tension measurements have been performed in various cell types, the tension distribution along the plasma membrane of motile cells has been largely unexplored. Here, we present an experimental study of the distribution of tension in the plasma membrane of rapidly moving fish epithelial keratocytes. We find that during steady movement the apparent membrane tension is ∼30% higher at the leading edge than at the trailing edge. Similar tension differences between the front and the rear of the cell are found in keratocyte fragments that lack a cell body. This front-to-rear tension variation likely reflects a tension gradient developed in the plasma membrane along the direction of movement due to viscous friction between the membrane and the cytoskeleton-attached protein anchors embedded in the membrane matrix. Theoretical modeling allows us to estimate the area density of these membrane anchors. Overall, our results indicate that even though membrane tension equilibrates rapidly and mechanically couples local boundary dynamics over cellular scales, steady-state variations in tension can exist in the plasma membranes of moving cells
Centering and symmetry breaking in confined contracting actomyosin networks
Centering and decentering of cellular components is essential for internal
organization of cells and their ability to perform basic cellular functions
such as division and motility. How cells achieve proper localization of their
components is still not well-understood, especially in large cells such as
oocytes. Here, we study actin-based positioning mechanisms in artificial cells
with persistently contracting actomyosin networks, generated by encapsulating
cytoplasmic Xenopus egg extracts into cell-sized water-in-oil droplets. We
observe size-dependent localization of the contraction center, with a symmetric
configuration in larger cells and a polar one in smaller cells. In the
symmetric state, the contraction center is actively centered, via a
hydrodynamic mechanism based on Darcy friction between the contracting network
and the surrounding cytoplasm. During symmetry breaking, transient attachments
to the cell boundary drive the contraction center to a polar location near the
droplet boundary. Our findings demonstrate a robust, yet tunable, mechanism for
subcellular localization
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Self-organized stress patterns drive state transitions in actin cortices
Biological functions rely on ordered structures and intricately controlled collective dynamics. This order in living systems is typically established and sustained by continuous dissipation of energy. The emergence of collective patterns of motion is unique to nonequilibrium systems and is a manifestation of dynamic steady states. Mechanical resilience of animal cells is largely controlled by the actomyosin cortex. The cortex provides stability but is, at the same time, highly adaptable due to rapid turnover of its components. Dynamic functions involve regulated transitions between different steady states of the cortex. We find that model actomyosin cortices, constructed to maintain turnover, self-organize into distinct nonequilibrium steady states when we vary cross-link density. The feedback between actin network structure and organization of stress-generating myosin motors defines the symmetries of the dynamic steady states. A marginally cross-linked state displays divergence-free long-range flow patterns. Higher cross-link density causes structural symmetry breaking, resulting in a stationary converging flow pattern. We track the flow patterns in the model actomyosin cortices using fluorescent single-walled carbon nanotubes as novel probes. The self-organization of stress patterns we have observed in a model system can have direct implications for biological functions
An Adhesion-Dependent Switch between Mechanisms That Determine Motile Cell Shape
Keratocytes are fast-moving cells in which adhesion dynamics are tightly coupled to the actin polymerization motor that drives migration, resulting in highly coordinated cell movement. We have found that modifying the adhesive properties of the underlying substrate has a dramatic effect on keratocyte morphology. Cells crawling at intermediate adhesion strengths resembled stereotypical keratocytes, characterized by a broad, fan-shaped lamellipodium, clearly defined leading and trailing edges, and persistent rates of protrusion and retraction. Cells at low adhesion strength were small and round with highly variable protrusion and retraction rates, and cells at high adhesion strength were large and asymmetrical and, strikingly, exhibited traveling waves of protrusion. To elucidate the mechanisms by which adhesion strength determines cell behavior, we examined the organization of adhesions, myosin II, and the actin network in keratocytes migrating on substrates with different adhesion strengths. On the whole, our results are consistent with a quantitative physical model in which keratocyte shape and migratory behavior emerge from the self-organization of actin, adhesions, and myosin, and quantitative changes in either adhesion strength or myosin contraction can switch keratocytes among qualitatively distinct migration regimes
Cell motility: the integrating role of the plasma membrane
The plasma membrane is of central importance in the motility process. It defines the boundary separating the intracellular and extracellular environments, and mediates the interactions between a motile cell and its environment. Furthermore, the membrane serves as a dynamic platform for localization of various components which actively participate in all aspects of the motility process, including force generation, adhesion, signaling, and regulation. Membrane transport between internal membranes and the plasma membrane, and in particular polarized membrane transport, facilitates continuous reorganization of the plasma membrane and is thought to be involved in maintaining polarity and recycling of essential components in some motile cell types. Beyond its biochemical composition, the mechanical characteristics of the plasma membrane and, in particular, membrane tension are of central importance in cell motility; membrane tension affects the rates of all the processes which involve membrane deformation including edge extension, endocytosis, and exocytosis. Most importantly, the mechanical characteristics of the membrane and its biochemical composition are tightly intertwined; membrane tension and local curvature are largely determined by the biochemical composition of the membrane and the biochemical reactions taking place; at the same time, curvature and tension affect the localization of components and reaction rates. This review focuses on this dynamic interplay and the feedbacks between the biochemical and biophysical characteristics of the membrane and their effects on cell movement. New insight on these will be crucial for understanding the motility process