Final answer:
Nitric oxide (NO) is a regulatory molecule that cannot enter its target cell. It acts as a paracrine chemical, diffusing across the plasma membrane to influence nearby smooth muscle cells and promote vasodilation. The correct option is C. nitric oxide
Step-by-step explanation:
The regulatory molecule that cannot enter its target cell is nitric oxide (NO).
Nitric oxide is a paracrine chemical that causes vasodilation, which means it relaxes and widens blood vessels. It is produced by endothelial cells, which are cells that line the inside of blood vessels. Nitric oxide acts as a messenger molecule, diffusing across the plasma membrane and influencing nearby smooth muscle cells, promoting vasodilation.
In contrast, molecules like epinephrine and acetylcholine can enter their target cells and bind to specific receptors on the plasma membrane, initiating cell signaling pathways and triggering specific responses within the cell. The correct option is C. nitric oxide