Now that we known about both prokaryotic and eukaryotic cells, let's look at the endosymbiosis theory. this theory suggests that mitochondria and plastids in eukaryotic cells were once independent prokaryotic cells. basically, this means that once upon a time, there were three prokaryotic cells: one that was capable of aerobic respiration and converting energy, one that was capable of photosynthesis and one that was incapable of doing either of these processes. The lazy cell that was incapable of doing respiration and photosynthesis engulfed - or ate - the other cells.