Final answer:
A multicellular organism functions through a hierarchical organization of cells forming tissues, which in turn form organs and organ systems; these subsystems interact to maintain life.
Step-by-step explanation:
A multicellular organism is a system of interacting subsystems, which include cells, tissues, organs, and organ systems. This hierarchical organization ensures that the organism functions efficiently and survives. Cells specialize to perform specific tasks, and similar cells form tissues, such as muscle or nervous tissue. These tissues collaborate to create organs, like the heart or lungs, each with a distinct role. Finally, related organs work together as part of an organ system, like the circulatory or respiratory system, to conduct essential bodily functions. This highly coordinated and structured approach enables an organism to respond to stimuli, maintain homeostasis, and support life at a larger scale than that of a single cell. The interaction between these subsystems facilitates complex biological processes, making life as a multicellular entity possible.