Final answer:
Nitric oxide (NO) is a toxic substance that acts as a ligand and can diffuse across the plasma membrane. It has various roles in the body, including inducing relaxation in smooth muscle and regulating blood pressure.
Step-by-step explanation:
Nitric oxide (NO) is a toxic substance. It is a gas that acts as a ligand and can diffuse directly across the plasma membrane. Nitric oxide has various roles in the body, including interacting with receptors in smooth muscle to induce relaxation and regulating blood pressure. It has a short half-life and is involved in the dilation of blood vessels, which is the mechanism of action for medications like Viagra.