Abstract

Cilia and flagella represent a best-seller of nature: These slender cell appendages propel sperm and many other microswimmers. Yet, cilia and flagella serve not only as “cell propeller”, but function also as sensory organelles. Thereby, they provide an ideal model system to study physical principles of motility control. In my talk, I will first address a specific example of chemo-sensation, showing how sperm find the egg by steering their path in response to chemical cues released by the egg. Recent experimental advances of tracking sperm cells navigating in three space dimensions allowed us to test a geometric theory of steering along helical paths, which we had put forward previously. In a second part, I will show how beating flagella also respond to mechanical forces. Collections of flagella can beat in synchrony, supposedly coupled by hydro-mechanical interactions. We combined theory and experiment to elucidate a mechanism of synchronization in the model organism Chlamydomonas: This tiny green alga swims like a breast-swimmer with two flagella, which can synchronize their beat . Our analysis shows how synchronization arises by a coupling of cell rocking and flagellar beating. We characterizes an exemplary force-velocity relationship of the flagellar beat: hydrodynamic forces set the speed of the beat. Remarkably, this novel synchronization mechanism operates independent of direct hydrodynamic interactions, and works reliably also in the presence of active flagellar fluctuations.