Which statement best describes the initial event in most cellular signaling pathways?

• A. The receptor is internalized after ligand binding.
• B. A second messenger is produced first.
• C. A ligand binds to its receptor to initiate signaling.
• D. The cell membrane depolarizes.

Answer: (C)

Cell signaling starts when a signal molecule, a ligand, binds to its receptor and activates it. That binding changes the receptor’s shape, turning it on so it can kick off the cascade of interactions inside the cell. Once the receptor is activated, it transmits the message through various routes—phosphorylation cascades, recruitment of signaling proteins, or production of second messengers like cAMP or Ca2+—to relay and amplify the signal to its targets.

The other options describe steps that happen after signaling has already begun or in specific contexts, not the initial trigger. Receptor internalization typically occurs after the signal has been received to attenuate or modulate the response. Second messengers are produced as part of the downstream signaling events, not the very first step. Membrane depolarization is specific to electrical signaling in excitable cells and isn’t the general starting event for most biochemical signaling pathways.