Prokaryotes: Prokaryotic Cells, Structure, Organization, and Facts
What are Prokaryotes
o Prokaryotes don’t have a membrane bound nucleus.
o They are split into two distinct domains called Bacteria and Archaea.
o Archaea have as much in common with eukaryotes as they do with bacteria.
o They are typically found in the extreme environments such as salty lakes and boiling hot springs.
o Unlike bacteria, the cell walls of archaea are not made from peptidoglycan.
o Most known prokaryotes are members of the domain Bacteria.
o The introduction of the two domains makes the kingdom Monera obsolete.
o The kingdom Monera was the kingdom containing all prokaryotes.
o In order to grow; all organisms require the ability to acquire carbon, energy and electrons (usually from hydrogen).
o Organisms can be classified according to the sources from which they gather these commodities.
o A carbon source can be organic or inorganic.
o Most carbon sources also contribute oxygen and hydrogen.
o CO2 is a unique inorganic carbon source because it has no hydrogens.
o To some degree, all microorganisms are capable of fixing CO2 (reducing it and using the carbon to create organic molecules usually through a process called the Calvin Cycle).
"Autotrophs are organisms that are capable of using CO2, as their sole source of carbon"
"Heterotrophs use preformed, organic molecules as their source of carbon"
Classifications
o Organisms can be classified as well according to how they get their energy:
1) Phototrophs use light as their energy source.
o Ex. Cyanobacteria: blue/green algae that uses photosynthesis.
o Only prokaryotes can acquire energy from an inorganic source other than light.
2) Chemotrophs use the oxidation of organic and inorganic matter as their energy source.
o Electrons or hydrogens can be acquired from inorganic matter by lithotrophs, or organic matter by organotrophs.
o You can recall this one because lithium is an inorganic molecule and lithotrophs get their hydrogens from inorganic molecules.
o Humans are chemoheterotrophs.
All organisms can be classified as one of each of the three types.
o Bacteria are found in all classifications.
o Some bacteria are capable of fixing nitrogen.
o Atmospheric nitrogen is abundant, but in a strongly bound form that is useless to plants.
o Nitrogen fixation is the process by which N2 is converted to ammonia.
o Most plants are unable to use ammonia however and must wait for other bacteria to further process the nitrogen in a process called nitrification.
o Nitrification is a two step process that creates NITRATES, which are useful to plants, from ammonia.
o Nitrification requires two genera (genus’s) of chemoautotrophic prokaryotes.
o Nitrification is the biological oxidation of ammonia with oxygen into nitrite followed by the oxidation of these nitrites into nitrates.
o Degradation of ammonia to nitrite is usually the rate limiting step of nitrification.
o Genus – (plural: genera) taxonomic group containing one or more species.
o Chemoautotrophy is an inefficient mechanism for acquiring energy, so chemoautotrophs require large amounts of substrate.
o All known chemoautotrophs are prokaryotes.
o Only prokaryotes can acquire energy from an inorganic source other than light.
Structure of Prokaryotes
o Prokaryotes have a single, circular double stranded molecule of DNA.
o This molecule is twisted into supercoils and is associated with histones in Archaea and with proteins that are different from histones in bacteria.
o The DNA, RNA and protein complex in prokaryotes forms a structure visible under the light microscope called a nucleoid (also called the chromatin body, nuclear region, or nuclear body).
o The nucleoid is not enclosed by a membrane.
Structure of Bacteria
o There are three major shapes of bacteria:
1) cocci (round)
2) bacilli (rod shaped)
3) spiral
o There are other shapes, including helical.
o Helically shaped bacteria are called spirilla, if they are rigid. Otherwise they are called spirochetes.
o Certain species of spirochetes may have given rise to eukaryotic flagella through a symbiotic relationship.
o The name of the bacteria often reveals the shape.
o Prokaryotes have no complex, membrane-bound organelles.
o The key words are ‘complex’ and ‘membrane bound’.
o They have organelles: ribosomes, nucleoids, and mesosomes etc….; just not complex membrane-bound organelles like mitochondria, ER, Golgi, etc.
o Prokaryotic ribosomes are smaller than eukaryotic ribosomes.
o They are made from a 30S subunit and a 50S subunit to form a 70S subunit.
o A prokaryote may or may not contain a mesosome.
o Mesosomes are invaginations of the plasma membrane and can be seen under the light microscope.
o Their exact function is unknown, but may be involved in cell wall formation during cell division.
o Prokaryotes also have inclusion bodies.
o Inclusion bodies are granules of organic or inorganic matter that may be visible under a light microscope.
o Inclusion bodies may or may not have a membrane.
Prokaryotes Citations
- Predation between prokaryotes and the origin of eukaryotes. Bioessays . 2009 Jul;31(7):748-57.
- From Prokaryotes to Cancer: Glutamine Flux in Multicellular Units. Trends Endocrinol Metab . 2017 Sep;28(9):637-644.
- Glycoproteins in prokaryotes. Arch Microbiol . 1997 Sep;168(3):169-75.
- Prokaryotes: the unseen majority. Proc Natl Acad Sci U S A . 1998 Jun 9;95(12):6578-83.
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