Final answer:
Alkaloids and terpenoids are distinct in their chemical structure, which is the primary way in which these plant defense mechanisms differ. Alkaloids contain nitrogen and have complex structures, while terpenoids are constructed from isoprene units. Both serve as chemical defenses against herbivores and pathogens.
Step-by-step explanation:
Plant Defense Mechanisms
Alkaloids and terpenoids are major types of plant defenses that protect plants against herbivores and pathogens. They differ primarily by their chemical structure. Alkaloids are nitrogen-containing compounds with complex structures that can affect animal physiology, including examples such as nicotine, morphine, and codeine. Terpenoids, on the other hand, are built from isoprene units and can also have potent effects on herbivores and pathogens. Both groups of compounds serve as chemical defenses, producing effects that range from deterring herbivores to causing severe health effects when ingested.
Their effectiveness and mode of action can be variable, so while they may differ in these aspects as well, their key distinction lies in the chemical structure. The origin of these defense compounds is plant metabolic pathways, although they vary in the specific pathways and precursors involved. Their variations in effectiveness and mode of action are secondary to the fundamental differences in structure, classifying them as distinct groups of secondary plant compounds.