Final answer:
Parasitism is a symbiotic relationship where one organism benefits at the expense of another. Biotrophs are a type of parasite that feed off living tissue. They have specialized structures called haustoria for nutrient acquisition.
Step-by-step explanation:
Parasitism is a symbiotic relationship where one organism, the parasite, benefits at the expense of another organism, the host. The parasite feeds on the host, weakening it by siphoning off its resources. However, the parasite usually doesn't kill the host quickly, as it needs time to complete its reproductive cycle. Biotrophs are a type of parasitic organism that feed off living tissue of their hosts, such as fungi like powdery mildews and rusts. Biotrophs produce structures called haustoria that penetrate the host's cells and access nutrients present in the cytosol. In summary, parasitic organisms harm their hosts, while biotrophs are a specific type of parasite that feed off living tissue and have specialized structures for nutrient acquisition.