Final answer:
The endosymbiotic theory culminates in a form of symbiosis known as mutualism, where both the host cells and the internalized bacteria benefit from the relationship.
Step-by-step explanation:
The form of symbiosis that the organisms in the endosymbiotic theory ended with is mutualism. This theory suggests that certain organelles, such as mitochondria and chloroplasts, were originally free-living bacteria that formed a mutually beneficial relationship with larger host cells, leading to the development of complex eukaryotic cells. In mutualism, both organisms benefit from the relationship, which is indicative of the theory wherein the host cell gained functionality such as ATP production or photosynthesis, while the bacteria gained a protected environment and stable nutrients. On the other hand, in commensalism, one organism benefits, and the other is unaffected, while in parasitism, one organism benefits at the detriment of the host. Endosymbiosis is a specific type of mutualism.