there are so many different kinds of living things that its so difficult to classify them as there would be too many classifications that it would become absurd to have only a few organsims in a single group. the role of classification is to organize as well as simplify so it proves to be a challenge when the subgroups branching from eukoryotes vs prokaryotes become too small and therefore it becomes a giant mess. even the current classification system (taxonomy) is flawed.
"Cellular respiration, the process by which organisms combine oxygen with foodstuff molecules, diverting the chemical energy in these substances into life-sustaining activities and discarding, as waste products, carbon dioxide and water."
