| Dynamic processes in cells are governed by the cell cytoskeleton, a network of elastic protein filaments. This gel-like network is inherently active, driven by force generating processes on the molecular scale such as the action of motor molecules. Of particular importance for cell mechanics and the control of cell shape is the cell cortex, a thin layer of actin cytoskeleon below the cell membrane. The cortical layer has a thickness of several hundred nanometers and filaments turn over within 30s. Therefore, on time-scales beyond 30s, the cortex can be viewed as a thin film of an active fluid which can be described by continuum theories. We will discuss the physics of such active films and present a quantitative theory for spontaneous flow patterns which are generated during cell division in the cortex by gradients of active stresses. Finally, we discuss the effects of chiral active processes in the filament network. Such chirality arises because filaments of the cytoskeleton are helicies. The interactions of motor molecules with such filaments gives rise to torque diploes in the fluid that lead to large scae flow patterns in the cell with chiral asymmetries. Such broken chiral symmetry has particularly rich effects near surfaces. Chirality in thin active films provides a key example for chiral asymmetries on the cellular scale. |
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