Video narration
Were mitochondria and chloroplasts always built into cells, like permanent fixtures-or did they move in like roommates? Endosymbiotic theory says the second is closer to the truth. Long ago, an archaeal-related host cell engulfed an oxygen-using α-proteobacterium. Instead of being digested, the two formed a lasting partnership: the host gained useful energy metabolism, while the bacterium gained protection and resources. Over generations, that bacterium gradually became a mitochondrion-the major site of aerobic energy metabolism in modern eukaryotic cells. That's the turn: a cell part began as an independent bacterium. Later, a mitochondria-containing eukaryotic cell engulfed a photosynthetic cyanobacterium. Again, the partnership lasted, and the cyanobacterium evolved into a chloroplast, enabling oxygenic photosynthesis-the ability to use light to make food and release oxygen. So, were they built in? No. Mitochondria and chloroplasts are bacterial descendants: ancient partners that became essential cell machinery.