Final answer:
The outer part of a G protein receptor binds to a ligand, while the inner part binds to a G-protein trimer.
Step-by-step explanation:
The outer part of a G protein receptor binds to a ligand, which is the effector signal molecule that activates the receptor. The inner part of the receptor binds to a G-protein trimer on the cytoplasmic surface of the plasma membrane.
When the G-protein trimer binds to the receptor, a GDP molecule associated with the a subunit of the G-protein is exchanged for GTP. This activates the G-protein and triggers a cellular response.
It's important to note that G-protein-linked receptors have seven transmembrane domains, but each receptor has its own specific extracellular domain and G-protein-binding site.