This document provides information about the levels of organization in the human body from cells to organ systems. It discusses: - Cells are the basic unit and there are two main types: multicellular organisms composed of more than one cell (like humans), and unicellular organisms composed of a single cell. - The levels of organization from simplest to most complex are: cells, tissues, organs, and organ systems. Cells combine to form tissues, tissues combine to form organs, and several organs working together form organ systems. - Stimuli are things that trigger responses, sensory organs detect stimuli and send messages via sensory nerves to the brain which coordinates responses. There are different types of responses including motor and glandular