What are the levels of organization in an animal's body, from least complex to most complex?

The correct sequence of organization in an animal's body starts at the most basic unit, the cell, and builds up to more complex structures. This structure follows a hierarchy that progresses from individual cells to tissues, which are groups of similar cells that perform a common function. Tissues then combine to form organs, which are structures composed of two or more types of tissues that cooperate to perform specific functions. Finally, organs come together to form organ systems, which carry out broad physiological roles.

This order reflects the increasing complexity and specialization of structures within the body, illustrating how simple units can organize into more intricate systems essential for survival and function. Understanding this hierarchy is fundamental to grasping how biological systems operate efficiently within an organism.