Final answer:
Phagocytosis is mainly utilized by cells in the animal immune system, such as macrophages and neutrophils, to defend against pathogens by engulfing and digesting them.
Step-by-step explanation:
Phagocytosis is a process where certain cells consume entire organisms, either as a means of feeding or as a defense against pathogens. Specifically, phagocytosis is a critical function of the innate immune response. The cells that primarily use phagocytosis include macrophages, neutrophils, and dendritic cells, which are all part of the animal immune system. These cells engulf pathogens into phagosomes, which then fuse with lysosomes containing digestive enzymes to destroy the pathogens. However, there are exceptions among microbes, such as Mycobacterium tuberculosis, which have developed mechanisms to evade destruction within phagocytes.
Absorptive feeding strategies seen in fungi and certain bacteria do not involve phagocytosis; instead, these organisms extract nutrients directly from their environment, such as plant tissues, often through structures like haustoria or by releasing enzymes to degrade surrounding material and absorb nutrients. Plants, in turn, have defense mechanisms against such herbivores and pathogens but do not consume organisms via phagocytosis.
Therefore, the answer to the student's question, "Who uses phagocytosis to consume entire organisms either as food or as their way of defending against pathogens?" is animals (option 2).