In what direction does the activation of a receptor propagate its signal?

• A. Sideways along the cell membrane
• B. From the extracellular to the intracellular domain
• C. From the intracellular domain to the extracellular
• D. Only within the nucleus

Answer: (B)

Activation starts when a ligand binds to the receptor’s extracellular domain, causing a conformational change that is transmitted across the membrane to the intracellular domain. This cross-membrane movement conveys the signal into the cell, where it triggers intracellular signaling cascades through second messengers or kinase activity, leading to a cellular response. Signals moving sideways along the membrane don’t deliver the message into the cell, and signaling from inside to outside or staying confined to the nucleus misses the essential step of crossing the membrane to reach intracellular signaling machinery.