A drop of dilute ammonium hydroxide is placed near paramecia. Which behavior is observed?

• A. They will stop moving.
• B. They will die immediately.
• C. They will all eventually swim away from the ammonium hydroxide.
• D. They will all swim toward the ammonium hydroxide.

Answer: (C)

Chemotaxis in single-celled organisms is the ability to move in response to chemical gradients. Paramecia can sense changes in their chemical environment and steer their swimming accordingly using their cilia. A drop of ammonium hydroxide creates a local, alkaline chemical gradient that is stressful to many aquatic microbes. In response, paramecia exhibit negative chemotaxis—they move away from the irritant source to seek a more favorable environment. Since the solution is only dilute, it isn’t instantly lethal, so the typical behavior is directed movement away rather than stopping or dying immediately, or moving toward the chemical.