Organisms With Nuclei
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Of the three domains of life, two are not eukaryotic: Bacteria and Archaea (Chapter 17). The third domain, Eukaryota, is the subject of this chapter and the following twenty chapters. The extra space given to this one domain is based more on our familiarity with the domain than on any sense of importance or diversity. The two prokaryotic domains likely have a far greater number of species but are on a much smaller scale than their human observers and are more difficult and/or less appealing to study. From what we have observed, their chemical diversity is much greater than that between organisms within the domain Eukaryota.
Many of the chemical cycles and cellular structures of the smallest, "lower" eukaryotic organisms are exactly the same as those of the highest plants and animals, including humans. All eukaryotes are characterized by having a nucleus in which the DNA is stored and generally have intracellular organelles such as mitochondria, chloroplasts, endoplasmic reticula, and others.
This chapter focuses on those eukaryotic organisms which do not fit into one of the three main kingdoms of the domain: Viridiplantae (Plants), Metazoa (Animals) and Fungi. In the five-kingdom system of classification which is still probably most widely used by the general public, this chapter would be called "Protista" or "Protoctista". Most of the organisms in this chapter are unicellular but some, like giant kelp, are multicellular and quite large.