  The present habilitation thesis in theoretical biological physics addresses
two central dynamical processes in cells and organisms: (i) active motility and
motility control and (ii) self-organized pattern formation. The unifying theme
is the nonlinear dynamics of biological function and its robustness in the
presence of strong fluctuations, structural variations, and external
perturbations. We theoretically investigate motility control at the cellular
scale, using cilia and flagella as ideal model system. Cilia and flagella are
highly conserved slender cell appendages that exhibit spontaneous bending
waves. This flagellar beat represents a prime example of a chemo-mechanical
oscillator, which is driven by the collective dynamics of molecular motors
inside the flagellar axoneme. We study the nonlinear dynamics of flagellar
swimming, steering, and synchronization, which encompasses shape control of the
flagellar beat by chemical signals and mechanical forces. Mechanical forces can
synchronize collections of flagella to beat at a common frequency, despite
active motor noise that tends to randomize flagellar synchrony. In Chapter 2,
we present a new physical mechanism for flagellar synchronization by mechanical
self-stabilization that applies to free-swimming flagellated cells. This new
mechanism is independent of direct hydrodynamic interactions between flagella.
Comparison with experimental data provided by experimental collaboration
partners in the laboratory of J. Howard (Yale, New Haven) confirmed our new
mechanism in the model organism of the unicellular green alga Chlamydomonas.
Further, we characterize the beating flagellum as a noisy oscillator. Using a
minimal model of collective motor dynamics, we argue that measured
non-equilibrium fluctuations of the flagellar beat result from stochastic motor
dynamics at the molecular scale. Noise and mechanical coupling are antagonists
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