Final answer:
Bitter taste is typically caused by alkaloids, which are basic nitrogen-containing compounds found in many plants and can affect G protein-coupled receptors on taste cells.
Step-by-step explanation:
The bitter taste is primarily elicited by alkaloids. Alkaloids are basic nitrogen-containing compounds found in various plant products like coffee, hops, tannins, tea, and aspirin. The presence of these toxic alkaloids makes the plants less susceptible to microbe infection and less appealing to herbivores. In the context of taste perception, bitter molecules can bind to G protein-coupled receptors on gustatory cells, which leads to various responses like depolarization or hyperpolarization, and changes in G protein activation within these cells.