What does it imply if a receptor is an integral component of the cell membrane?

• A. It spans the membrane and has extracellular and intracellular domains.
• B. It is located only inside the nucleus.
• C. It floats in cytosol with no membranes.
• D. It is secreted outside the cell.

Answer: (A)

An integral membrane receptor is embedded in the lipid bilayer and spans the membrane, having parts both outside and inside the cell. The extracellular portion binds signaling molecules, the transmembrane region anchors the receptor in the membrane, and the intracellular portion interacts with signaling proteins to relay the message inside the cell. This arrangement allows the signal from outside the cell to trigger responses inside. Other options describe receptors that are entirely inside the nucleus, dissolved in the cytosol, or secreted outside the cell, none of which involve a membrane-spanning architecture with distinct extracellular and intracellular domains.