Categorizing the different classes of microorganisms (microbes) allows scientists to easily characterize, compare, and differentiate between the trillions of microbes that colonize Earth. All life is organized into three domains—Archaea, Bacteria, and Eukarya—that are differentiated based on similarities and differences in DNA sequence and biochemical properties. Archaea and Bacteria are single prokaryotic cells that lack a nucleus and generally lack organelles but can have external structures such as flagella and pili for motility and attachment. Eukarya are organisms with eukaryotic cells containing a nucleus and other membrane-bound components. Examples of Eukarya include protists, fungi, parasitic worms, plants, and animals. Eukaryotes can be unicellular or multicellular, and they have organelles with various functions. Eukaryotic cells and prokaryotic cells make proteins from instructions in their genetic material, with several key differences in how they perform this function.
At A Glance
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Scientific classification separates organisms into categories based on relatedness; to avoid confusion, scientists use a binomial naming system consisting of the organism's genus and species.
- The domains Bacteria and Archaea are made up of prokaryotic organisms, while Eukarya contains eukaryotic organisms, including protists, fungi, parasitic worms, plants, and animals.
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Prokaryotes are simple, single-celled organisms that do not have a nucleus or other membrane-bound organelles.
- Prokaryotes contain free-floating DNA and plasmids and can have external structures such as pili and flagella that are used for attachment and motility.
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Bacterial cell walls are made of peptidoglycan, which can be used to distinguish between different classes of bacteria.
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Eukaryotes are unicellular or multicellular organisms that have cells with a distinct nucleus and membrane-bound organelles.
- The grouping Protista consists of unicellular algae, protozoa, and slime molds, all with unique life cycles and behaviors.
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Fungi can be multicellular or unicellular, and they reproduce by releasing spores, budding, or fragmentation.
- The helminths include tapeworms, flukes, and roundworms and species in each of these groups are significant parasites of larger organisms, including humans.
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Eukaryotes have a distinct nucleus containing many chromosomes, while prokaryotes have a single chromosome contained in an area called a nucleoid.
- Both prokaryotes and eukaryotes use ribosomes to build protein but rely on different factors and sequences to accomplish this.
