Final answer:
The portion of the transmembrane receptor that binds to hydrophilic messengers is called the ligand-binding domain.
Step-by-step explanation:
The portion of the transmembrane receptor that binds to hydrophilic messengers is termed the ligand-binding domain.
Each cell-surface receptor has three main components: an external ligand-binding domain, a hydrophobic transmembrane domain, and an intracellular domain. The ligand-binding domain, also known as the extracellular domain, is responsible for binding to hydrophilic ligands such as hormones or neurotransmitters.