Final answer:
Caffeine is the alkaloid that gives it its bitter taste and protects it from insects.
Step-by-step explanation:
The alkaloid that gives caffeine its bitter taste and protects it from hungry insects is Caffeine. Caffeine is a stimulant found in coffee, tea, and some soft drinks. It acts as a neurotransmitter, blocking the activity of adenosine, which carries messages across nerve cells.