What do cells regulate to ensure signals are transient?

• A. The activity of transducer molecules
• B. The nucleus
• C. The mitochondria
• D. The cytoskeleton

Answer: (A)

The main idea is that signals are kept transient by quickly turning off the transducer molecules that carry the message from the receptor to the rest of the cell. These transducers—like G proteins, kinases, and second messengers—serve as the active conduits of signaling, so their activity is tightly regulated to stop the signal fast. Mechanisms include hydrolysis of GTP to GDP on G proteins, inactivation of kinases by phosphatases, and breakdown or removal of second messengers such as cAMP or IP3. Calcium is cleared from the cytosol by pumps and sequestration, and receptors can desensitize to prevent further signal amplification. Because these controls act directly on the signaling intermediates, they determine how long the signal lasts. In contrast, the nucleus responds later by changing gene expression, mitochondria are more about energy and apoptosis, and the cytoskeleton is involved in structural responses rather than timing of the signal.