Endosymbiosis is the idea that cell organelles evolved from prokaryotic organisms that originally lived inside larger cells, eventually losing the ability to function as independent organisms. Because the organelles found within eukaryotic cells share a number of similarities with bacteria—such as mitochodria and chloroplasts—many scientists believe that early versions of eukaryotic cells had symbiotic relationships with certain bacteria. In particular, the eukaryote provided protection and resources, while the prokaryote specialized in converting energy (either sunlight or chemical) into forms that could be used by the eukaryotic cell (sugar or ATP, or adenosine triphosphate)— thus the term endosymbiote, meaning “shared internal life.” In addition, structural similarities exist between chloroplasts, mitochondria, and free-living bacteria. (For more about ATP and energy, see the chapter “Basics of Biology.”)