That some Bacteria require an oxygen-free environment harkens back to the earth's earliest times and suggest their ancient origin. The Kingdom Animalia is comprised of multi-celled organisms which develop from an embryo resulting from the fertilization of an egg by a much smaller sperm. The Kingdom Plantae is composed of multi-celled organisms that grow from embryos that are usually the result of sexual fusion of a male and female cell. Again there are exceptions although somewhere in every plant's past, there were sexual forbears. As a result, plants are the great producers of life.
Plants generally have a rigid cell wall composed of cellulose.
Therefore, some argue that fungi are more closely related to animals than plants. Fungi develop from spores without any embryonic stage. They digest other living things outside their bodies by releasing enzymes and then absorbing the product. Kingdom Protoctista is the catch-all kingdom for everything that does not fit into the other four. The five kingdom model is not universally accepted.