Thesis (pdf) - Espci

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4 1 General introduction Finally, in the third chapter of the thesis we consider the interaction between the cellular membrane and the cytoskeletal cortex. The plasma membrane and the cytoskeletal cortex are amongst the most important parts of the cell. They participate in providing cell shape, separate the extracellular media from the cytosol, act as mechanotransducers, and are responsible for cell motion. The adhesion between cortex and membrane is thus fundamental for the proper functionality of a cell. Failure or weakening of this adhesion induces a phenomenon named blebbing, which consist of highly dynamical spherical protrusions that nucleate as a consequence of membrane unbinding and retract with the appearance of a new cortex under the bare membrane. The properties of bleb formation and membrane-cortex adhesion can be experimentally addressed using mainly micropipette aspiration. We propose both a kinetic and energetic model for cortex and membrane adhesion which we use to understand the dependence of the adhesion strength on the active state of the cell and external perturbations such as osmotic shocks or micropipette aspirations. Once addressed the concept of adhesion energy, we consider the dynamical response of the membrane and cortex to a mechanical perturbation. In particular, we focus on the treadmilling properties of the cortex and its viscoelastic response using a Maxwell like description for the short time elastic and long time viscous behavior of the cortex. Using the micropipette aspiration set up as a model system, we theoretically identify all the possible aspiration regimes and we compare them with both existing experimental measurements, and experiments we have conducted on the Entamoeba histolytica system in collaboration with B. Mauguis and F. Amblart.
2 Probing elastic anisotropy from defect dynamics in Langmuir Monolayers 2.1 Introduction The study of topological defects is of general relevance in many areas of physics and biology, ranging from cosmology to superfluid helium or cell division. An important motivation arises from the universality of the defects, which occur in any system with order parameter. Their main properties are independent of the underlying physics, determined only by symmetries, the dimension of the order parameter and the defect charge. This universality has been lately exploited by condensed matter systems such as liquid crystals which allow to perform relatively easy experiments yielding knowledge in completely different physical areas. Of particular interest is the study of the motion of a defect, and the interaction and annihilation dynamics of defect pairs, which have been extensively studied by several groups (Ryskin and Kremenetsky, 1991; Svenˇsek and ˇZumer, 2002; Blanc et al., 2005). A remarkable feature of defect pairs annihilation, both observed experimentally and numerically, although not fully understood, is the enhanced mobility of the positively charged defect with respect to its negative counterpart. Elastic anisotropy (inequality of the elastic constants) and hydrodynamic effects arising from defect motion (backflow) have been shown to generically contribute to explain this asymmetry. In fact, the latter is dominant in the context of bulk (3-d) liquid crystals (Tóth et al., 2002; Svenˇsek and ˇZumer, 2002; Oswald and Ignés-Mullol, 2005; Blanc et al., 2005), thus hindering the possibility to develop a simple method to quantitatively relate material elasticity to defect dynamics, which would be an interesting alternative to traditional methods to determine the elastic constants (de Gennes and Prost, 1995; Oswald and Ignés-Mullol, 2005).
Page 1: Studies of dynamical phenomena in s
Page 5 and 6: Acknowledgements This thesis has be
Page 7 and 8: Contents 1 General introduction . .
Page 9: Contents v 4.6 Conclusions . . . .
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5 General conclusions In this work
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5 General conclusions 115 number of
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A Resum en català El present treba
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A.1 Auto-organització i cooperativ
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A.2 Mesura de l’anisotropia elàs
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A.3 Interaccions de membrana i còr
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A.3 Interaccions de membrana i còr
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B Résumé en Français Ce travail
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B.1 Auto-organisation et coopérati
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B.2 Mesure de anisotropie élastiqu
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B.3 Interactions de membrane et cor
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B.4 Conclusions 135 une perturbatio
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References Alberts, B., Johnson, A.
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REFERENCES 139 Evans, E. (2001). Pr
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REFERENCES 141 Kruse, K., Joanny, J
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REFERENCES 143 Sit, P., Spector, A.
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