Eukaryotic cells originated approximately __ years ago. They differ from prokaryotic cells in that they contain a membrane-bound __ , in addition to other organelles. The nucleus is thought to have evolved due to an invagination of the __ . The __ , organelles that synthesize ATP, and the __ , organelles that synthesize glucose, are thought to have evolved from __ . The __ suggests that these __ were engulfed by another cell. However, instead of being consumed, a __ occured and they became organelles. This theory of the origin of organelles is called the __ .