Thesis (pdf) - Espci

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2 1 General introduction gies. In this context, out of equilibrium statistical physics is a good candidate in trying to model processes at this microscopic scale. At a more coarse grained level, the cell can be characterized by its shape and mechanical properties. Traditionally, the physics of the state of the matter (condensed matter), dealt with the macroscopic states of the matter that arise from the physical properties of the microscopic constituents (atoms or molecules) namely, solid and liquid. However, the state of cells and other materials such as polymers, colloids, foams or gels, do not correspond either to liquid or solid. They have a large degree of ordering and are definitely not simple liquids. Furthermore, they are fairly soft materials that can be deformed by thermal fluctuations. The ”softness” of these materials lead to a recent field in physics named soft matter. The typical energy scale for cell membrane deformation is of κ ∼ 10 − 100kBT , which is slightly larger than thermal fluctuations. Soft-matter is a good candidate to study coarse grained macroscopic properties of the cell where thermal fluctuations are averaged although play a role in determining shape and mechanical properties. At cellular scales, inertial forces are negligible when compared to viscous forces (the Reynolds number Re ≪ 1), and Aristotelian physics, where velocity and force are linearly related, govern generically cellular and sub-cellular phenomena (molecular motor motion, intracellular transport, cell deformation). In recent years, many qualitative advances in experimental techniques, ranging from microscopy, micromanipulation to gene transfection, have led to an enormous quantity of new data and observations at the cellular level which require fundamentally new concepts to help their understanding. In this context, physicists and biologists live an exciting period where physical concepts can help to describe and understand new general cellular properties, which at the same time can be directly accessed experimentally. Soft-matter and statistical physics are the obvious candidates to bridge between biology and physics, although new physical and biological tools will probably emerge as new phenomena is observed. My personal point of view is that the present state in biophysical research is qualitative similar to that in the early years of particle physics, where the first particle accelerators led to the discovering of new fundamental particles. The large amount of interesting and fundamental problems to be understood, the ease of experimental comparison, the beauty to relate simple concepts such as geometry, topology, or elasticity to biological phenomena, and the personal and scientific enrichment of the interdisciplinary work involved,
1 General introduction 3 led me, irremediably, to devote this thesis and my future research to biophysics. This thesis do not respond to a pre-established scheme, although there is a logic underlying the pattern followed, which starts from a string theory PhD student during the first two years of my doctorate, and ends as a biophysicist. The transition between such a perpendicular fields has resulted in the following thesis, where aspects of purely soft matter concerning topological defects appear first, followed by a study of some cooperativity of molecular motors, and ends with a purely biophysical approach of membrane-cytoskeleton interactions which explicitly connects with biological experiments. Yet from the beginning I tried to work on problems that allowed me to interact with experiments and researchers from other fields. This thesis is composed of three chapters which are devoted to soft-matter and physical biology. The first chapter belongs to the field of topological defects. The study of topological defects is of general relevance in many areas of physics and biology, because their main universal properties can help understand systems that presumably exhibit topological defects, such as the mitotic spindle during cell division, using simple condensed matter experiments, like in liquid crystals. We consider the motion of defects and their interaction, which is not fully understood. In this regard, we study the dynamics of annihilation of two defects in Langmuir monolayers using a liquid crystal model. We are able to extract the dependence of the elastic anisotropy of the material on the surface pressure measuring only the ratio of velocities at which the defects approach each other. In the second chapter of the thesis, we consider some aspects of selforganization and cooperativity of molecular motors. Molecular motors are proteins that convert chemical energy into mechanical work at a molecular scale, and are responsible of many biological phenomena, ranging from muscle contraction to cellular transport and cell motility. Molecular motors usually cooperate to generate large forces, as in the case of muscle contraction. Although there are several theoretical works considering the coupling between molecular motors, there is not yet a full understanding of cooperativity and force distribution among clusters of molecular motors. We consider several representative weakly interactions between motors, namely, short range and long range repulsion, and weak attraction, and we study their collective behavior, force distribution and efficiency when a force is applied in the foremost motor. Our approach is mainly based on Langevin simulations using a two state model for a molecular motor.
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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Page 16 and 17: 6 2 Probing elastic anisotropy from
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52 4 Membrane-cortex interactions:
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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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