G Protein-Coupled Receptors: These are a type of receptor protein that activate G proteins when they bind to a substance, initiating an intracellular signaling cascade.
Ligand:This is any molecule that binds specifically to a receptor site of another molecule. In our analogy, this would be the guest with the right ID.
Transmembrane Protein: This is any protein that spans the entire biological membrane. Receptor proteins are often transmembrane proteins as they need to interact with both inside and outside environments.