Biomedical illustration of G protein coupled receptors. These vary in size and function, span the cell membrane, and are activated by a variety of ligands, including hormones, neurotransmitters, odors, pheromones and even light. These adrenergic receptors (or adrenoceptors) are targets of the catecholamines, especially noradrenaline (norepinephrine) and adrenaline (epinephrine). Here an epinephrine molecule binds to a GPCR causing a conformational change which activates an associated G protein. The G protein consists of three parts, each of which may have a separate function within the cell. Because they are involved in many diseases, they are targeted by roughly 50% of modern medicinal drugs.