Prokaryotes: Prokaryotic Cells, Structure, Organization, and Facts

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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"

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


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