Final answer:
A single ligand like norepinephrine can trigger various cellular responses because it binds to different types of adrenergic receptors, initiating distinct signaling pathways in different cells.
Step-by-step explanation:
The fact that a single ligand such as norepinephrine can lead to several types of cellular responses is best explained by its ability to bind to different adrenergic receptors, which are present on the plasma membrane of various cells.
When norepinephrine binds to alpha-adrenergic and beta-adrenergic receptors, it activates different signaling pathways inside the cell.
For example, when norepinephrine binds to beta-adrenergic receptors, it activates a G-protein, which in turn activates adenylyl cyclase.
This leads to the conversion of ATP to cyclic AMP (cAMP), a second messenger that mediates cell-specific responses like glycogenolysis and gluconeogenesis. The enzyme phosphodiesterase eventually breaks down cAMP, which terminates the signal.
Different types of cells have different combinations of adrenergic receptors and associated signaling pathways, which accounts for the variety of physiological effects that norepinephrine can have, including stimulating the nervous system, increasing heart rate, dilating pupils, and mobilizing energy.