327 research outputs found

    Giant vesicles at the prolate-oblate transition: A macroscopic bistable system

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    Giant phospholipid vesicles are shown to exhibit thermally activated transitions between a prolate and an oblate shape on a time scale of several seconds. From the fluctuating contour of such a vesicle we extract ellipticity as an effective reaction coordinate whose temporal probability distribution is bimodal. We then reconstruct the effective potential from which we derive an activation energy of the order of kBTk_BT in agreement with theoretical calculations. The dynamics of this transition is well described within a Kramers model of overdamped diffusion in a bistable potential. Thus, this system can serve as a model for macroscopic bistability.Comment: 10 pages, LaTeX, epsfig, 4 eps figures included, to appear in Europhys. Let

    Phosphorus-acquisition strategies of canola, wheat and barley in soil amended with sewage sludges

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    Crops have different strategies to acquire poorly-available soil phosphorus (P) which are dependent on their architectural, morphological, and physiological root traits, but their capacity to enhance P acquisition varies with the type of fertilizer applied. The objective of this study was to examine how P-acquisition strategies of three main crops are affected by the application of sewage sludges, compared with a mineral P fertilizer. We carried out a 3-months greenhouse pot experiment and compared the response of P-acquisition traits among wheat, barley and canola in a soil amended with three sludges or a mineral P fertilizer. Results showed that the P-acquisition strategy differed among crops. Compared with canola, wheat and barley had a higher specific root length and a greater root carboxylate release and they acquired as much P from sludge as from mineral P. By contrast, canola shoot P content was greater with sludge than with mineral P. This was attributed to a higher rootreleased acid phosphatase activity which promoted the mineralization of sludge-derived P-organic. This study showed that contrasted P-acquisition strategies of crops allows increased use of renewable P resources by optimizing combinations of crop and the type of P fertilizer applied within the cropping system

    How to determine local elastic properties of lipid bilayer membranes from atomic-force-microscope measurements: A theoretical analysis

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    Measurements with an atomic force microscope (AFM) offer a direct way to probe elastic properties of lipid bilayer membranes locally: provided the underlying stress-strain relation is known, material parameters such as surface tension or bending rigidity may be deduced. In a recent experiment a pore-spanning membrane was poked with an AFM tip, yielding a linear behavior of the force-indentation curves. A theoretical model for this case is presented here which describes these curves in the framework of Helfrich theory. The linear behavior of the measurements is reproduced if one neglects the influence of adhesion between tip and membrane. Including it via an adhesion balance changes the situation significantly: force-distance curves cease to be linear, hysteresis and nonzero detachment forces can show up. The characteristics of this rich scenario are discussed in detail in this article.Comment: 14 pages, 9 figures, REVTeX4 style. New version corresponds to the one accepted by PRE. The result section is restructured: a comparison to experimental findings is included; the discussion on the influence of adhesion between AFM tip and membrane is extende

    Effective Area-Elasticity and Tension of Micro-manipulated Membranes

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    We evaluate the effective Hamiltonian governing, at the optically resolved scale, the elastic properties of micro-manipulated membranes. We identify floppy, entropic-tense and stretched-tense regimes, representing different behaviors of the effective area-elasticity of the membrane. The corresponding effective tension depends on the microscopic parameters (total area, bending rigidity) and on the optically visible area, which is controlled by the imposed external constraints. We successfully compare our predictions with recent data on micropipette experiments.Comment: To be published in Phys. Rev. Let

    Active Membrane Fluctuations Studied by Micropipet Aspiration

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    We present a detailed analysis of the micropipet experiments recently reported in J-B. Manneville et al., Phys. Rev. Lett. 82, 4356--4359 (1999), including a derivation of the expected behaviour of the membrane tension as a function of the areal strain in the case of an active membrane, i.e., containing a nonequilibrium noise source. We give a general expression, which takes into account the effect of active centers both directly on the membrane, and on the embedding fluid dynamics, keeping track of the coupling between the density of active centers and the membrane curvature. The data of the micropipet experiments are well reproduced by the new expressions. In particular, we show that a natural choice of the parameters quantifying the strength of the active noise explains both the large amplitude of the observed effects and its remarkable insensitivity to the active-center density in the investigated range. [Submitted to Phys Rev E, 22 March 2001]Comment: 14 pages, 5 encapsulated Postscript figure

    Implications des relations sol-plante en ingénierie écologique des habitats et sols métallifères dégradés: le cas des habitats riches en cuivre du Katanga (République Démocratique du Congo)

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    International audienceLa restauration écologique des habitats dégradés et des sols nus riches en métaux lourds créés par l'activité minière est devenue aujourd'hui un enjeu environnemental majeur pour réduire l'érosion de la biodiversité et la dégradation des paysages, des sols et de l'eau. Les études de restauration écologique basées sur l'identification des espèces sans faire référence aux traits fonctionnels des plantes sont limitées au réservoir régional des espèces et rendent difficile les interprétations et les applications dans des contextes écologiques variés. En comparant les traits fonctionnels des plantes entre l'habitat métallifère non dégradé et un gradient de succession d'habitats secondaires, il est possible de définir des traits liés à la capacité de colonisation des sols nus riches en métaux lourds. Cette première tentative d'analyse des traits sur les habitats riches en métaux lourds permet ainsi d'identifier les traits candidats pour la phytoremediation. Un des futurs challenges est de créer de nouveaux écosystèmes fonctionnels sur les sols nus contaminés par les métaux lourds e
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