The eucaryotic cell originated from a symbiosis of several bacterial species. They thus distinguish themselves from the much smaller and simpler prokaryotes , cells without internal structure. Bacteria are prokaryotes, while protozoans, like the Amoeba, and the cells of fungi, plants and animals are eukaryotes. According to the endosymbiotic theory of Lynn Margulis , organelles originated as smaller bacteria-like cells that were engulfed by a larger cell and continued to live inside that cell, using its products while producing some other products in return.
Mitochondria are bacteria that became specialized as energy producers.
This is a transmission electron micrograph of rat liver cells. Part of the nucleus is in the right lower corner. Circular objects are mitochondria. Biological Evolution Up Prev.