Jurnal Edukasi
Not a member yet
4879 research outputs found
Sort by
Paramecium swimming and ciliary beating patterns: a study on four RNA interference mutations
No full textInternational audienceParamecium cells swim and feed by beating their thousands of cilia in coordinated patterns. The organization of these patterns and its relationship with cell motility has been the subject of a large body of work, particularly as a model for ciliary beating in human organs where similar organization is seen. However the rapid motion of the cells makes quantitative measurements very challenging. Here we provide detailed measurements of the swimming of Paramecium cells from high-speed video at high magnification, as they move in microfluidic channels. An image analysis protocol allows us to decouple the cell movement from the motion of the cilia, thus allowing us to measure the ciliary beat frequency (CBF) and the spatio-temporal organization into metachronal waves along the cell periphery. Two distinct values of the CBF appear at different regions of the cell: most of the cilia beat in the range of 15 to 45 Hz, while the cilia in the peristomal region beat at almost double the frequency. The body and peristomal CBF display a nearly linear relation with the swimming velocity. Moreover the measurements do not display a measurable correlation between the swimming velocity and the metachronal wave velocity on the cell periphery. These measurements are repeated for four RNAi silenced mutants, where proteins specific to the cilia or to their connection to the cell base are depleted. We find that the mutants whose ciliary structure is affected display similar swimming to the control cells albeit with a reduced efficiency, while the mutations that affect the cilia's anchoring to the cell lead to strongly reduced ability to swim. This reduction in motility can be related to a loss of coordination between the ciliary beating in different parts of the cell
Inertial versus baroclinic instability of the Bickley jet in continuously stratified rotating fluid
No full textInternational audienceThe paper contains a detailed study of the inertial instability of a barotropic Bickley jet on the f-plane in the continuously stratified primitive equations model, and a comparison of this essentially ageostrophic instability with the classical baroclinic one. Analytical and numerical investigation of the linear stability of the jet in the long-wave sector is performed for a range of Rossby and Burger numbers. The major results are that: (1) the standard symmetric inertial instability, appearing at high enough Rossby numbers, turns out to be the infinite-wavelength limit of an asymmetric inertial instability, this latter having the highest growth rate for a large range of vertical wavenumbers; (2) inertial instability coexists with the standard baroclinic instability, which becomes dominant at small Rossby numbers. Nonlinear saturation of the inertial instability of the jet with a superimposed random small-amplitude perturbation is then studied, using the Weather Research and Forecast model. It is shown that at first stages the inertial instability dominates. It is localized near the maximum of the anticyclonic shear and is associated with the highest attainable value of the vertical wavenumber. The saturation of the inertial instability leads to the homogenization of the geostrophic momentum in the unstable region. At later stages, another baroclinic instability develops, characterized by lower values of the vertical wavenumber. This instability saturates by forming large-scale vortices downstream. It is identified as the leading instability of a marginally inertially stable jet resulting from the initial one through homogenization of the geostrophic momentum. The rough scenario of the evolution of essentially ageostrophic jets is, thus, as follows: the inertial instability rapidly saturates and baroclinic instability takes over. It is shown that reorganization of the flow due to developing instabilities is an efficient source of inertia-gravity waves
Changes in rainfall regime over Burkina Faso under the climate change conditions simulated by 5 regional climate models
No full textInternational audienceSahelian rainfall has recorded a high variability during the last century with a significant decrease (more than 20 %) in the annual rainfall amount since 1970. Using a linear regression model, the fluctuations of the annual rainfall from the observations over Burkina Faso during 1961–2009 period are described through the changes in the characteristics of the rainy season. The methodology is then applied to simulated rainfall data produced by five regional climate models under A1B scenario over two periods: 1971–2000 as reference period and 2021–2050 as projection period. As found with other climate models, the projected change in annual rainfall for West Africa is very uncertain. However, the present study shows that some features of the impact of climate change on rainfall regime in the region are robust. The number of the low rainfall events (0.1–5 mm/d) is projected to decrease by 3 % and the number of strong rainfall events ([50 mm/d) is expected to increase by 15 % on average. In addition, the rainy season onset is projected by all models to be delayed by one week on average and a consensus exists on the lengthening of the dry spells at about 20 %. Furthermore, the simulated relationship between changed annual rainfall amounts and the number of rain days or their intensity varies strongly from one model to another and some changes do not correspond to what is observed for the rainfall variability over the last 50 years
Experimental and numerical study of crack healing in a nuclear glass
No full textInternational audiencea b s t r a c t An experimental study of thermally or water-induced crack healing in an inactive borosil-icate glass, chemically analogous to that used in France for the vitrification of nuclear waste was carried out. Partial welding of glass plates was observed after annealing in air at 425 °C (77 °C below T g) when at least 20 MPa compressive stress was applied, while annealing at 450 °C under 20 MPa led to a complete disappearance of the interface. Closure of indenta-tion-induced cracks was observed during annealing at 400 °C in an ESEM as a result of viscous relaxation of residual stresses but it did not constitute a sufficient proof of crack healing. DCDC specimens were thus pre-cracked in an ESEM and then either annealed at various temperatures (350–490 °C) in secondary vacuum or in air, or left in water at 70–90 °C, sometimes under a compressive stress normal to the crack face. The specimens were then reloaded in the ESEM and the crack opening displacements under a given load were compared to those measured during pre-cracking. The cracks were bridged by an alteration layer over a distance from the crack tip which decreased as loading increased. The restraining effect of these bridges on crack opening was assessed via finite element simulations, using interface elements. The tensile strength of the bridging layer was estimated as 27–39 MPa after vacuum annealing at 400 °C, 11–20 MPa after 15 days in water at 90 °C and 44–78 MPa after 11 days in water at 70 °C under 5 MPa normal compression. Partially healed cracks did not resume propagation from their former crack tip, but due to branch cracks re-initiated a few hundred microns behind it which grow avoiding the healed area. This behaviour was explained using finite element simulations
Determination of collagen fibril size via absolute measurements of second-harmonic generation signals.
No full textInternational audienceThe quantification of collagen fibril size is a major issue for the investigation of pathological disorders associated with structural defects of the extracellular matrix. Second-harmonic generation microscopy is a powerful technique to characterize the macromolecular organization of collagen in unstained biological tissues. Nevertheless, due to the complex coherent building of this nonlinear optical signal, it has never been used to measure fibril diameter so far. Here we report absolute measurements of second-harmonic signals from isolated fibrils down to 30 nm diameter, via implementation of correlative second-harmonic-electron microscopy. Moreover, using analytical and numerical calculations, we demonstrate that the high sensitivity of this technique originates from the parallel alignment of collagen triple helices within fibrils and the subsequent constructive interferences of second-harmonic radiations. Finally, we use these absolute measurements as a calibration for ex vivo quantification of fibril diameter in the Descemet's membrane of a diabetic rat cornea
Ancestral lineages and limit theorems for branching Markov chains
No full textWe consider a branching model in discrete time for structured population in varying environment. Each individual has a trait, which belongs to some general state space and both the reproduction law and the trait inherited by the offsprings may depend on the trait of the mother and the environment.We study the long time behavior of the population and the ancestral lineage of typical individuals under general assumptions. We focus on the growth rate and the trait distribution among the population for large time and provide some estimations of the local densities. A key role is played by well chosen (possibly non-homogeneous) Markov chains. It relies in particular on an extension of many-to-one formula and the analysis of the genealogy, in the vein of the spine decomposition. The applications use the spectral gap of the mean operator, the Harris ergodicity or the large deviations of this auxiliary Markov chain
Copper-Catalyzed Coupling of N-Tosylhydrazones with Amines: Synthesis of Fluorene Derivatives
No full textInternational audienceAn original formation of one C-N bond and one C-C bond on the same carbenic center has been developed. This approach involves a copper-catalyzed cross-coupling reaction between 2’-bromo-biaryl-N-tosylhydrazones and different amines leading to 9H-fluoren-9-amine derivatives. This reaction proceeds under mild conditions in glycerol, an inexpensive and environmentally friendly solvent without adding any external ligand
A bit of tropical geometry
No full textInternational audienceThis friendly introduction to tropical geometry is meant to be accessible to first year students in mathematics. The topics discussed here are basic tropical algebra, tropical plane curves, some tropical intersections, and Viro's patchworking. Each definition is explained with concrete examples and illustrations. To a great exten, this text is an updated of a translation from a french text by the first author. There is also a newly added section highlighting new developments and perspectives on tropical geometry. In addition, the final section provides an extensive list of references on the subject
