A drop of dilute ammonium hydroxide placed near paramecia: which behavior will be observed?

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

Answer: (A)

Paramecia move by beating cilia and can sense chemical gradients in their environment. They tend to move toward nutrients (attractants) and away from harmful substances (repellents). A drop of dilute ammonium hydroxide creates a chemical gradient with higher concentration near the source, which is interpreted as a repellent cue. In response, the paramecia reorient and swim away from the higher concentration region. This directed movement is why they would eventually all move away from the ammonium hydroxide source, rather than stopping, moving randomly, or being attracted toward it.