Final answer:
An example of an alkaloid is nicotine, which, along with morphine, codeine, and heroin, has significant effects on human physiology and can act as a defense mechanism for plants against pests.
Step-by-step explanation:
An example of an alkaloid is nicotine, which is found in tobacco plants. Alkaloids are nitrogen-containing basic compounds that are extracted from plants. These substances are often referred to as secondary plant compounds that serve as chemical defenses against pests. Alkaloids include a wide variety of compounds known for their physiological and psychotropic effects on humans, serving various purposes in medicine as anesthetics, antidepressants, and stimulants, while some are habit-forming or even toxic. Other well-known examples besides nicotine include morphine, codeine, and heroin, which are derived from the opium poppy. These alkaloids are capable of causing profound changes in the body, ranging from relief of pain to addiction and, in excessive amounts, to respiratory depression and death.