Symbiotic association means one of the organisms, called endosymbiont, is present inside the cells of its host. The most typical examples are endosymbiosis theory of mitochondria and chloroplasts which, according to some theories, are cellular organelles of bacterial origin. But these theories are still discussed because there are in fact arguments that support endosymbiosis theory and also arguments that disapprove it.
Here are the answer for the exercise explaining the endosymbiosis A ==> oxygen-using procaryote (before being a mitochondrion) B ==> mitochondrion C ==> Nucleus D ==> endoplasmic reticulum E ==> Host cell: ancestor of all eucaryotes F ==> photosynthetic procaryote (before being a chloroplast) G ==> chloroplast H ==> Host cell: Ancestor for photosynthetic eucaryotes
