Cilia are tiny hairs protruding from cells that wave mucus and remove toxins from the respiratory tract. Cigarette smoke damages cilia, preventing mucus clearance and causing smoker's cough. Flagella are tails powered by molecular motors that propel some prokaryotes by alternating between forward movement and tumbling, while others have a single rotating flagellum for locomotion. Chemotaxis is cellular movement toward or away from chemicals, with positive chemotaxis referring to movement toward a liked chemical.