ASCENDING COMPLEXITY. “Cells can then be organized into tissues, which form organs, and organs can be grouped into organ systems, which are built into entire organisms—including humans like you. Organisms are units that can form populations, and then biospheres, which go on to make up even greater levels of complexity.”
http://www.nature.com/scitable/topicpage/biological-complexity-and-integrative-levels-of-organization-468