Figure 13.3  Figure 13.3 The structure of 7TM receptors. (A) Schematic representation of a 7TM receptor showing how it passes through the membrane seven times. (B) Three-dimensional structure of a subtype (β2) of the β-adrenergic receptor, a 7TM receptor taking part in fuel mobilization. Notice the ligand-binding site near the extracellular surface.