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.
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