A drop of dilute ammonium hydroxide is placed near paramecia. They will eventually all

• A. swim toward the ammonium hydroxide
• B. show no net change in behavior
• C. be instantly killed by the ammonium hydroxide
• D. swim away from the ammonium hydroxide

Answer: (D)

Paramecia exhibit chemotaxis, moving toward or away from chemical signals in their environment. A drop of ammonium hydroxide creates a chemical gradient that can be irritating or toxic to these cells by raising the pH and introducing ammonia. Faced with this unfavorable gradient, paramecia detect the change and steer away from the higher concentration of the irritant. So, they will swim away from the ammonium hydroxide as a protective response. They wouldn’t be attracted to it, stay inactive, or die instantly from a dilute exposure.