Efficacy, and not potency is how much is done. Efficacy measurement reuires that enough ligand is given that further ligand addition will not increase the response . Two curves of the same efficacy but different potencies B. Relation between the binding to a receptor and action. Receptor occupancy theory. It must be somewhat true, since if you don't have any binding of receptor, not much is going to happen, and there is less of an effect if a small amount of receptors are bound than if all receptors are bound.
Some drugs can bind and to the same site and yet have different maximal activity.
Some just don't want to work. The structure of the receptor has to allow it to function. Membrane bound with the receptor binding region on the outside of the membrane. Receptors that have similarities in structure are related. The use of techniques of molecular biology have opened up investigation into receptor structure and revolutionized receptor identification and classification.