Chemotaxis is defined as what?

• A. The movement of an organism in response to a chemical stimulus
• B. The movement of an organism toward or away from light
• C. The movement of water in response to osmotic pressure
• D. The random movement of organisms with no stimuli

Answer: (A)

Chemotaxis is the directed movement of an organism in response to a chemical stimulus. An organism senses a chemical gradient and moves toward higher concentrations of a beneficial chemical (positive chemotaxis) or away from a harmful one (negative chemotaxis). This is why bacteria swim toward nutrients and immune cells migrate toward signaling molecules at infection sites. The other options describe movement in response to different cues (light), to osmotic changes, or random motion, none of which involve navigating a chemical gradient.