Prokaryotic organisms comprise two of the three domains of organisms.
• Bacteria (sometimes called Eubacteria)
• Formerly, these two groups were placed in the kingdom Monera.
• We will look into the differences between the two domains a bit later. But first let's concentrate first on features common to all prokaryotes.
All archaea and bacteria have prokaryotic cells.
• Prokaryotic cells are fundamentally different from eukaryotic cells.
• Prokaryotes originated about 2 billion years before eukaryotes.
• Obviously, prokaryotes should be separated from eukaryotes at a high taxonomic level (domain)
• Recent molecular evidence suggests that although both archaea and bacteria are prokaryotic, they are different enough to be placed in different domains.
Prokaryotes are simpler in structure than eukaryotes.
• Most prokaryotes are basically unicellular.
• In some species cells aggregate into "colonies," but there is rarely any cytoplasmic connection between cells, and usually all cells in the colony are of the same type.
• Prokaryotic cells have no membrane-bound organelles (such as the nuclei, mitochondria, and chloroplasts that are found in eukaryotic cells).
• However, photosynthetic bacteria (such as cyanobacteria) have internal membranes similar to the thylakoids of eukaryotes, but these membranes are not enclosed within a chloroplast.
• DNA is not surrounded by a membrane (no nucleus).
• The region containing the DNA is called the nucleoid.
• Prokaryotic DNA is "naked" (without histone proteins)
• Prokaryotic ribosomes are smaller (70s) than those of eukaryotes.
• Bacteria look very simple-even when seen with the electron microscope.
|A bacterium cell seen with the transmission electron microscope. Note the plasma membrane, cell wall, and nucleoid (the lighter area)|
Most prokaryotes have a cell wall (like algae, fungi, and plants).
• Cell walls of archaea are built mainly of protein.
• Cell walls of bacteria are built mainly of peptidoglycan.
• Peptidoglycan is a mesh-like molecule composed of chains of sugars cross-linked by short chains of amino acids.
• This is very different from the walls of eukaryotes, which are built mainly from polysaccharides.
• Some bacteria also have a lipopolysaccharide (LPS) layer.
• The LPS is located outside of the peptidoglycan layer.
• It is somewhat like the lipid bilayer of the plasma membrane.
• There are two groups of bacteria based on the cell wall.
• These groups are distinguished by the reaction of their cells to a purple dye (crystal violet). Called the Gram stain, it was developed by Hans Gram in the 1800s. Two types of bacteria can be identified by their color after the treatment.
• Gram-positive bacteria
• Appear purple after staining
• They retain the purple dye because of their thick peptidoglycan layer and lack of the LPS.
• Gram-negative bacteria
• Appear pink after staining
• They do not retain the purple dye.
• Their peptidoglycan layer is thin, and they have the LPS.
• The cell wall is often surrounded by a mucilaginous sheath.
• This sheath is sometimes so thick that the cells appear to be embedded in a mass of jelly.
Prokaryotes are vitally important ecologically and economically.
• Prokaryotes are found in virtually every place suitable for life.
• They are the most numerous of all organisms.
• Most bacteria are important as decomposers.
• Cyanobacteria are important producers in aquatic ecosystems.
• Bacteria carry out reactions that influence the environment, especially with regard to the nitrogen cycle.
• Some parasitic bacteria cause diseases.
• Many archaea live in extreme environments
• Thermophiles - in hot springs
|Mammoth hot springs in Yellowstone National Park. The coloration is due to thermophilic bacteria and cyanobacteria|
Photograph by John Tiftickjian
• Halophiles - in very salty conditions
Structure and motility
• Prokaryotic cells are typically spherical (coccus), rod-shaped (bacillus), or helical (spirillum).
|Typical shapes of bacterial cells|
• Daughter cells separate shortly after cell division
• Cells remain attached in clumps after cell division
• This often happens because cells are surrounded by a sticky mucilage sheath (capsule)
|Two cyanobacteria illustrating colony and filamentous morphologies|
Micrographs by John Tiftickjian
• Cells remain attached after division.
• Cells always divide in the same plane.
• Cells are joined in long strands.
• Some species swim by means of flagella.
• Bacterial flagella are composed of a solid protein rod.
• Eukaryotic flagella have a complex structure of microtubules.
• Bacterial flagella are analogous to flagella of eukaryotes (not homologous). Although superficially similar, and they give the cell the ability to move, they are very different structurally.
• Cyanobacteria never have flagella.
• Some can move by "gliding" (mostly cyanobacteria)
• Reproduction is only asexual (no gametes).
• Unicellular species reproduce by cell division.
• Cell division is typically by binary fission.
• No mitosis (there is no nucleus)
• Chromosomes still must replicate and be separated between the daughter cells, but this works by attachment of chromosomes to the plasma membrane, there is no spindle.
• Colonies and filaments reproduce by fragmentation.
• Pieces of a colony or filament make break off and continue to grow, creating new individuals.
• Some species can exchange genes through conjugation.
• Cells connect to each other by a pilus.
• DNA transfers from one cell to the other through the pilus.
• This is different from true sexual reproduction because gametes are not formed (there is no meiosis).
Nutrition and metabolism
• Bacteria differ in their requirement for oxygen
• Obligate anaerobes - killed by oxygen
• Obligate aerobes - must have oxygen
• Can use oxygen when it is present, but can also live anaerobically in oxygen-poor environments.
• Prokaryotes can obtain energy in a variety of ways
• Some are heterotrophic (require organic food).
• Saprobes feed on dead organic matter (decomposers)
• Parasites feed on living organisms (causing diseases)
• Some are symbiotic with eukaryotic organisms.
• Some are autotrophic (make their own food).
• Chemotrophic bacteria
• These utilize reactions of inorganic compounds to provide energy to make organic food molecules.
• Photosynthetic bacteria
• Cyanobacteria do photosynthesis using chlorophyll a (like algae and plants)
• This produces oxygen as a byproduct (again, like algae and plants)
• There are other photosynthetic bacteria that lack chlorophyll a and do not produce oxygen.
• Many can carry out specialize metabolic pathways such as nitrogen fixation.
• Nitrogen fixation converts N2 gas from the atmosphere into ammonium (NH4+) ions.
• Ammonium combines with organic compounds to form amino acids.
• No eukaryotes can use N2 directly from the atmosphere.
• Cyanobacteria do this in special cells called heterocysts.
|Heterocysts in the cyanobacterium Anabaena|
Micrograph by John Tiftickjian
• Symbiotic relationships
• Many prokaryotes live symbiotically (mutualism) with eukaryotes
• Rhizobium lives within roots of certain plants (especially legumes).
• They live in special root nodules
• They fix nitrogen which the plant benefits from.
• They in turn obtain food from the plant.
• Since oxygen inactivates the nitrogen fixing enzyme, the root nodules act as "anaerobic chambers."
• Some cyanobacteria partner with fungi to form lichens.
Classification of the prokaryotes is based mainly on chemical traits.
• Domain Archaea
• Have very unusual metabolic features
• Cell walls lack a true peptidoglycan component
• Membranes contain some unusual lipids (diglycerols)
• Nucleotide sequences of ribosomal RNA different than for other prokaryotes
• Grow in exotic environments
• extremely high salinity - halophiles
• strongly acidic environments - acidophiles
• high temperatures - thermophiles
• Domain Bacteria or Eubacteria (true-bacteria)
• Sometimes called blue-green algae
• Used in this sense, algae is a common name. Cyanobacteria are not closely related to other algae.
• All true algae are eukaryotic, classified in the Protista.
• The blue-greens are special in that they are almost the only prokaryotes that contain chlorophyll a.
• Because chlorophyll a is found in all algae and all higher plants, it's likely that some ancient cyanobacterium was the ancestor of the chloroplast.
• Remember that the chloroplast probably evolved through endosymbiosis.
• Many other groups (see textbook for more examples)
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