Step 1: Understanding the Concept:
Alkaloids are naturally occurring organic compounds that contain at least one nitrogen atom. Morphine is the principal alkaloid derived from the opium poppy (\textit{Papaver somniferum}).
Step 2: Detailed Explanation:
1. Natural Origin: Morphine is extracted directly from opium, making it a natural alkaloid.
2. Chemical Structure: It contains a complex ring system based on phenanthrene. This distinguish it from other opium alkaloids like papaverine, which belong to the benzylisoquinoline class.
3. Opioid classification: While it is an opioid, it is the parent natural molecule from which semi-synthetic opioids (like Heroin) and synthetic ones (like Fentanyl) are modeled.
Step 3: Final Answer:
Morphine is a natural alkaloid characterized by its phenanthrene nucleus.