168 research outputs found
Fluctuations of the Casimir-like force between two membrane inclusions
Although Casimir forces are inseparable from their fluctuations, little is
known about these fluctuations in soft matter systems. We use the membrane
stress tensor to study the fluctuations of the membrane-mediated Casimir-like
force. This method enables us to recover the Casimir force between two
inclusions and to calculate its variance. We show that the Casimir force is
dominated by its fluctuations. Furthermore, when the distance d between the
inclusions is decreased from infinity, the variance of the Casimir force
decreases as -1/d^2. This distance dependence shares a common physical origin
with the Casimir force itself.Comment: 5 pages, 3 figure
Relational EPR
We study the EPR-type correlations from the perspective of the relational
interpretation of quantum mechanics. We argue that these correlations do not
entail any form of 'non-locality', when viewed in the context of this
interpretation. The abandonment of strict Einstein realism implied by the
relational stance permits to reconcile quantum mechanics, completeness,
(operationally defined) separability, and locality.Comment: Revised, published versio
Interplay of packing and flip-flop in local bilayer deformation. How phosphatidylglycerol could rescue mitochondrial function in a cardiolipin-deficient yeast mutant
In a previous work, we have shown that a spatially localized transmembrane pH gradient, produced by acid micro-injection near the external side of cardiolipin-containing giant unilamellar vesicles, leads to the formation of tubules that retract after the dissipation of this gradient. These tubules have morphologies similar to mitochondrial cristae. The tubulation effect is attributable to direct phospholipid packing modification in the outer leaflet, that is promoted by protonation of cardiolipin head-groups. In this study, we compare the case of cardiolipin-containing giant unilamellar vesicles with that of giant unilamellar vesicles that contain phosphatidylglycerol (PG). Local acidification also promotes formation of tubules in the latter. However, compared with cardiolipin-containing giant unilamellar vesicles the tubules are longer, exhibit a visible pearling, and have a much longer lifetime after acid micro-injection is stopped. We attribute these differences to an additional mechanism that increases monolayer surface imbalance, namely inward PG flip-flop promoted by the local transmembrane pH gradient. Simulations using a fully nonlinear membrane model as well as geometrical calculations are in agreement with this hypothesis. Interestingly, among yeast mutants deficient in cardiolipin biosynthesis, only the crd1-null mutant, which accumulates phosphatidylglycerol, displays significant mitochondria! activity. Our work provides a possible explanation of such a property and further emphasizes the salient role of specific lipids in mitochondria! function. n a previous work, we have shown that a spatially localized transmembrane pH gradient, produced by acid micro-injection near the external side of cardiolipin-containing giant unilamellar vesicles, leads to the formation of tubules that retract after the dissipation of this gradient. These tubules have morphologies similar to mitochondrial cristae. The tubulation effect is attributable to direct phospholipid packing modification in the outer leaflet, that is promoted by protonation of cardiolipin headgroups. In this study, we compare the case of cardiolipin-containing giant unilamellar vesicles with that of giant unilamellar vesicles that contain phosphatidylglycerol (PG). Local acidification also promotes formation of tubules in the latter. However, compared with cardiolipin-containing giant unilamellar vesicles the tubules are longer, exhibit a visible pearling, and have a much longer lifetime after acid micro-injection is stopped. We attribute these differences to an additional mechanism that increases monolayer surface imbalance, namely inward PG flip-flop promoted by the local transmembrane pH gradient. Simulations using a fully nonlinear membrane model as well as geometrical calculations are in agreement with this hypothesis. Interestingly, among yeast mutants deficient in cardiolipin biosynthesis, only the crd1-null mutant, which accumulates phosphatidylglycerol, displays significant mitochondrial activity. Our work provides a possible explanation of such a property and further emphasizes the salient role of specific lipids in mitochondrial function
Forces exerted by a correlated fluid on embedded inclusions
We investigate the forces exerted on embedded inclusions by a fluid medium
with long-range correlations, described by an effective scalar field theory.
Such forces are the basis for the medium-mediated Casimir-like force. To study
these forces beyond thermal average, it is necessary to define them in each
microstate of the medium. Two different definitions of these forces are
currently used in the literature. We study the assumptions underlying them. We
show that only the definition that uses the stress tensor of the medium gives
the sought-after force exerted by the medium on an embedded inclusion. If a
second inclusion is embedded in the medium, the thermal average of this force
gives the usual Casimir-like force between the two inclusions. The other
definition can be used in the different physical case of an object that
interacts with the medium without being embedded in it. We show in a simple
example that the two definitions yield different results for the variance of
the Casimir-like force.Comment: 15 pages, 2 figure
Membranes by the Numbers
Many of the most important processes in cells take place on and across
membranes. With the rise of an impressive array of powerful quantitative
methods for characterizing these membranes, it is an opportune time to reflect
on the structure and function of membranes from the point of view of biological
numeracy. To that end, in this article, I review the quantitative parameters
that characterize the mechanical, electrical and transport properties of
membranes and carry out a number of corresponding order of magnitude estimates
that help us understand the values of those parameters.Comment: 27 pages, 12 figure
Monsters, Laws of Nature, and Teleology in Late Scholastic Textbooks
In the period of emergence of early modern science, âmonstersâ or individuals with physical congenital anomalies were considered as rare events which required special explanations entailing assumptions about the laws of nature. This concern with monsters was shared by representatives of the new science and Late Scholastic authors of university textbooks. This paper will reconstruct the main theses of the treatment of monsters in Late Scholastic textbooks, by focusing on the question as to how their accounts conceived natureâs regularity and teleology. It shows that they developed a naturalistic teratology in which, in contrast to the naturalistic explanations usually offered by the new science, finality was at central stage. This general point does not impede our noticing that some authors were closer to the views emerging in the Scientific Revolution insofar as they conceived nature as relatively autonomous from God and gave a relevant place to efficient secondary causation. In this connection, this paper suggests that the concept of the laws of nature developed by the new science âas exception-less regularitiesâtransferred to natureâs regularity the âstrongâ character that Late Scholasticism attributed to finality and that the decline of the Late Scholastic view of finality played as an important concomitant factor permitting the transformation of the concept of laws of nature
Membrane-mediated interactions
Interactions mediated by the cell membrane between inclusions, such as
membrane proteins or antimicrobial peptides, play important roles in their
biological activity. They also constitute a fascinating challenge for
physicists, since they test the boundaries of our understanding of
self-assembled lipid membranes, which are remarkable examples of
two-dimensional complex fluids. Inclusions can couple to various degrees of
freedom of the membrane, resulting in different types of interactions. In this
chapter, we review the membrane-mediated interactions that arise from direct
constraints imposed by inclusions on the shape of the membrane. These effects
are generic and do not depend on specific chemical interactions. Hence, they
can be studied using coarse-grained soft matter descriptions. We deal with
long-range membrane-mediated interactions due to the constraints imposed by
inclusions on membrane curvature and on its fluctuations. We also discuss the
shorter-range interactions that arise from the constraints on membrane
thickness imposed by inclusions presenting a hydrophobic mismatch with the
membrane.Comment: 38 pages, 10 figures, pre-submission version. In: Bassereau P., Sens
P. (eds) Physics of Biological Membranes. Springer, Cha
Recommended from our members
Mathematical analysis of the Escherichia coli chemotaxis signalling pathway
We undertake a detailed mathematical analysis of a recent nonlinear ordinary differential equation (ODE) model describing the chemotactic signalling cascade within an {\it Escherichia coli} cell. The model includes a detailed description of the cell signalling cascade and an average approximation of the receptor activity. A steady-state stability analysis reveals the system exhibits one positive real steady-state which is shown to be asymptotically stable. Given the occurrence of a negative feedback between phosphorylated CheB (CheB-P) and the receptor state, we ask under what conditions, the system may exhibit oscillatory type behaviour. A detailed analysis of parameter space reveals that whilst variation in kinetic rate parameters within known biological limits is unlikely to lead to such behaviour, changes in the total concentration of the signalling proteins does. We postulate that experimentally observed overshoot behaviour can actually be described by damped oscillatory dynamics and consider the relationship between overshoot amplitude, total cell protein concentration and the magnitude of the external ligand stimulus. Model reductions of the full ODE model allow us to understand the link between phosphorylation events and the negative feedback between CheB-P and receptor methylation, as well as elucidate why some mathematical models exhibit overshoot and others do not. Our manuscript closes by discussing intercell variability of total protein concentration as means of ensuring the overall survival of a population as cells are subjected to different environments
- âŠ