This thesis presents a study of the elasticity of three very different cytoskeletal materials, microtubules (MTs), filamentous actin (F-actin) and vimentin, one of the intermediate filaments (IFs). Using bulk rheology, multiple particle tracking, confocal microscopy, and transmission electron microscopy (TEM), we study the microscopic origin of the elasticity of these cytoskeletal networks.;In Chapter 1, we briefly introduce the properties of the three components of cytoskeletal filaments as well as the rheology essential to provide the background and motivation of this thesis. In Chapter 2, we describe the materials and experimental techniques involved.;In Chapter 3, we study solutions of purified MTs, as well as networks permanently cross-linked with biotin-NeutrAvidin. We show that the mechanical properties of MT solutions cannot be explained by the non-interacting rigid rod model. Instead, they show behavior very similar to the permanently cross-linked networks, suggesting the presence of effective cross-linking even in pure MT solutions. We develop a simple model based on transient cross-linking interactions between MTs to interpret the rheological response. We also calculate a lower bound estimate of the strength of this interaction.;In Chapter 4, we investigate the mechanical response of the composite networks of F-actin and MTs. We find that even a small concentration of MTs leads to dramatic and qualitative changes in the elastic properties of F-actin networks. MTs provide a way to regulate the nonlinear stiffening response of F-actin. Theoretically this can be understood in terms of an inhomogeneity in the strain field of the gel. This finding is highly relevant for interpretation of the mechanical behavior of the intracellular cytoskeleton, in which a dilute network of MTs coexists with a denser meshwork of more flexible biopolymers such as F-actin.;In Chapter 5, we study the third and final filamentous protein of the cytoskeleton, intermediate filaments (IFs). We find that divalent ions act as cross- linkers in vimentin networks. We demonstrate that the linear and nonlinear elastic responses of vimentin IF networks at intermediate stress can be quantitatively explained by stretching the entropic fluctuations of single semiflexible filaments; at high stress, we propose that enthalpic stretching of the individual filaments contributes to the observed nonlinear response.
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