1. What are the major characteristics of each of the three domains of life? Why do some researchers believe that the Kingdom Protista should be split into more than one kingdom? In the early days of biology, all organisms were classified as either plants or animals. Single-celled eukaryotic organisms such as algae and Paramecium were assigned to one of the kingdoms according to whether they are photosynthetic or not. Fungi and prokaryotes were grouped with plants, because fungi are sedentary and prokaryotes have cell walls. A five-kingdom scheme was devised because some organisms don't fit well into a two-kingdom system of classification. The simple cells of prokaryotes set them apart in their own kingdom, Monera. Organisms in the other four kingdoms all have eukaryotic cells. Fungi are not photosynthetic, so they are given their own kingdom. The kingdom Protista is an assortment of eukaryotes that don't fit into the other kingdoms-- mostly unicellular organisms such as Paramecium, unicellular algae, and multicellular algae.
The five-kingdom scheme divides living things into two fundamentally different groups-- prokaryotes and eukaryotes. New data suggests that two kinds of prokaryotes diverged early in the history of life, and thus there are actually three major categories of organisms. A new classification system assigns more significance to the ancient split by creating a level of classification larger than a kingdom-- a new category called a domain. The two domains of prokaryotes are Bacteria and Archaea. The domain Eukarya encompasses all of the kingdoms of eukaryotes. Taxonomists are in the process of sorting out various kingdoms of prokaryotes within the domains Bacteria and Archaea, and also splitting the protists into several kingdoms within the domain Eukarya.
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