What does the endosymbiont theory explain about the origins of plant and animal cell

The endosymbiont theory explain that the mitochondrian and endoplasmic recticlum present in plant and animal are descendant of some symbiosis bacteria

This is what i found on google “phytoplankton, tiny organisms that live in the ocean, are autotrophs. some types of bacteria are autotrophs. most autotrophs use a process called photosynthesis to make their food.” i hope this you for what your looking for