Final answer:
Mitochondria form when a primitive cell engulfs an aerobic bacterium, supporting the endosymbiotic theory.
Explanation:
When a primitive cell engulfs an aerobic bacterium, mitochondria will form. Mitochondria are known as the 'powerhouse of the cell' and are the site of cellular respiration. They originated from the endosymbiotic event where a free-living aerobic prokaryotic cell was engulfed by an anaerobic cell. The structure of mitochondria, with its double membranes and inner membrane infoldings called cristae, supports the endosymbiotic theory.
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