chemical: atoms and molecules (chapter 2) cellular: cells and their organelles (chapter 3) tissue: groups of similar cells (chapter 4) epithelium, muscle, connective tissue, nervous organ: contains two or more types of tissues organ system: organs that work closely together organismal: all organ systems **add picture receptor receives a stimuli, control center that processes and integrates information from the receptor, and the effector that responds to the control center to change the body's response. a collection of cells working together to perform a specific function