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Eukaryotic cells are some 10,000 times larger than prokaryotic cells by volume, have their DNA organised in a nucleus, and contain membrane-bound organelles. The division between prokaryotes and eukaryotes has been considered the most important distinction or difference among organisms.
Combined with the five-kingdom model, this created a six-kingdom model, where the kingdom Monera is replaced by the kingdoms Bacteria and Archaea. [16] This six-kingdom model is commonly used in recent US high school biology textbooks, but has received criticism for compromising the current scientific consensus. [ 13 ]
Historically, bacteria were considered a part of the Plantae, the plant kingdom, and were called "Schizomycetes" (fission-fungi). [169] For this reason, collective bacteria and other microorganisms in a host are often called "flora". [170] The term "bacteria" was traditionally applied to all microscopic, single-cell prokaryotes.
The other three kingdoms in his system were the eukaryotic Fungi, Animalia, and Plantae. Whittaker, however, did not believe that all his kingdoms were monophyletic. [25] Whittaker subdivided the kingdom into two branches containing several phyla: Myxomonera branch Cyanophyta, now called Cyanobacteria; Myxobacteria; Mastigomonera branch Eubacteriae
Chloroplasts have many similarities with cyanobacteria, including a circular chromosome, prokaryotic-type ribosomes, and similar proteins in the photosynthetic reaction center. [205] [206] The endosymbiotic theory suggests that photosynthetic bacteria were acquired (by endocytosis) by early eukaryotic cells to form the first plant cells ...
Originally, Linnaeus established three kingdoms in his scheme, namely for Plants, Animals and an additional group for minerals, which has long since been abandoned. Since then, various life forms have been moved into three new kingdoms: Monera, for prokaryotes (i.e., bacteria); Protista, for protozoans and most algae; and Fungi.
The three-domain system adds a level of classification (the domains) "above" the kingdoms present in the previously used five- or six-kingdom systems.This classification system recognizes the fundamental divide between the two prokaryotic groups, insofar as Archaea appear to be more closely related to eukaryotes than they are to other prokaryotes – bacteria-like organisms with no cell nucleus.
The monera differs from the four other kingdoms as "members of the Monera have a prokaryotic cytology in which the cells lack membrane-bound organelles such as chloroplasts, mitochondria, nuclei, and complex flagella." [1] The bacteria can be divided into two major subkingdoms: Eubacteria and Archaebacteria.