This document discusses the levels of organization of living things from cells to organisms. It explains that cells are the basic unit of structure and function, and that multicellular organisms have cells that specialize and divide labor. Tissues are groups of similar cells that work together, organs are made of different tissues and work together, and organ systems are groups of organs that work together. The highest level of organization is the organism, which is a complete living thing. Examples of organ systems in humans are provided like the digestive, circulatory, respiratory, reproductive and excretory systems.