What part of a transmembrane receptor binds to hydrophilic messengers?

The correct answer identifies the ligand-binding domain as the part of a transmembrane receptor that specifically interacts with hydrophilic messengers, such as neurotransmitters or hormones. This domain is typically located on the extracellular side of the membrane, allowing it to effectively engage with these water-soluble molecules.

Hydrophilic messengers cannot easily cross the lipid bilayer of the membrane due to their polarity. Therefore, the receptor must have a specialized area, the ligand-binding domain, designed to recognize and bind these signaling molecules. Upon binding, the receptor undergoes a conformational change that often triggers a cascade of intracellular events, facilitating communication and response within the cell.

Other domains play different roles; for instance, the transmembrane domain is crucial for anchoring the receptor within the membrane, while the intracellular domain typically interacts with signaling pathways or other cellular machinery. The receptor-binding domain term is somewhat overlapping but does not specify the ligand-binding function as clearly as the ligand-binding domain does. Thus, the ligand-binding domain is most accurately described as the component that engages hydrophilic messengers.