By this point, you may be tired of hearing about mitochondria and chloroplasts. Enough already, right? Well, we could lie and say that there is no more, but, well… we'd be lying. Sorry. Don’t give up now, though, because the most interesting part (we swear) about these seemingly benign organelles is yet to come. We promise, you won’t be disappointed. (This promise is not associated with any monetary refund.)
Our story begins many, many eons ago..."yeeeeeeah, back when the world was new, the planet Earth was down on its luck." Oh, sorry. We had a bit of a Hercules flashback there. Carry on.
We begin our story many eons ago, probably not long after the first eukaryotic cells evolved. At some point, during the earliest stages of life on Earth, a eukaryotic cell named Eukie engulfed, through the process of phagocytosis, a prokaryotic cell named Prokie capable of converting chemical energy into ATP. Eukie did not destroy the engulfed Prokie, but instead, the two got along swimmingly. Eukie provided a plethora of sugars for Prokie, and Prokie provided a substantial amount of energy for Eukie. A match made in heaven. Why isn't there a Disney movie about this yet?
Through time, Prokie divided again and again until there were many daughter prokaryotes living inside Eukie. OK, maybe this is why this story is not a Disney movie… When Eukie replicated, she passed some of Prokie's daughter cells onto her descendents. The energy from the prokaryotes gave these new eukaryotes a huge advantage, and more of them survived than almost any other eukaryotes on the planet. This story is a little weird, but so far so good…
Through time, nearly every other eukaryotic cell on Earth went extinct (poor guys!) while the eukaryote carrying the prokaryote thrived. Way to go, Eukie and Prokie. Along the way, one of the thriving eukaryotes ingested yet another prokaryote (getting weird again…), this one capable of converting sunlight into sugar. Therefore, through time, some eukaryotic cells had both types of prokaryotes, and some had only the first type of prokaryote. Those that had both types no longer had to spend time looking for food, but instead could focus on producing food through the process of photosynthesis.
Although it is impossible to know for sure how all the details played out—our story above is a bit contrived, for sure—biologists are fairly certain that the ancestors of both mitochondria and chloroplasts were originally bacteria that had been engulfed by primitive eukaryotes. Some of the evidence for this idea, called the Endosymbiotic Theory, is that both mitochondria and chloroplasts have their own DNA that codes for some of the proteins necessary for cellular respiration and photosynthesis. This DNA is circular, just like that seen in prokaryotic cells.
In addition, multiple membranes surround both mitochondria and chloroplasts, a structure only seen in prokaryotes. Both mitochondria and chloroplasts divide by binary fission, or the splitting of a single cell into two nearly equal daughter cells, just like bacteria, and both contain ribosomes that are structurally similar to prokaryotic ribosomes. Last but not least, the thylakoid structure in chloroplasts is a structure only seen in cyanobacteria. These, and quite a few other lines of evidence, lead the vast majority of biologists to accept the Endosymbiotic Theory as an accurate explanation the presence of mitochondria and chloroplasts in eukaryotic cells. It is truly a fascinating case study in the evolutionary relationship between prokaryotic and eukaryotic cells.