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Directional movement depends on the configuration of the flagella. Bacteria can move in response to a variety of environmental signals, including light ( phototaxis ), magnetic fields ( magnetotaxis ) using magnetosomes, and, most commonly, chemical gradients ( chemotaxis ). Purposeful movement toward a chemical attractant, like a food source, or away from a repellent, like a poisonous chemical, is achieved by increasing the length of runs and decreasing the length of tumbles . When running, flagella rotate in a counterclockwise direction, allowing the bacterial cell to move forward. In a peritrichous bacterium, the flagella are all bundled together in a very streamlined way ( [link] ), allowing for efficient movement. When tumbling, flagella are splayed out while rotating in a clockwise direction, creating a looping motion and preventing meaningful forward movement but reorienting the cell toward the direction of the attractant. When an attractant exists, runs and tumbles still occur; however, the length of runs is longer, while the length of the tumbles is reduced, allowing overall movement toward the higher concentration of the attractant. When no chemical gradient exists, the lengths of runs and tumbles are more equal, and overall movement is more random ( [link] ).

A diagram showing the run and tumble of bacterial motion. The tumble is when a clockwise rotation of flagella cause the bacterial cell to tumble about. The run is when a counter-clockwise rotation of the flagella cause the bacterial cell to move in a linear direction.
Bacteria achieve directional movement by changing the rotation of their flagella. In a cell with peritrichous flagella, the flagella bundle when they rotate in a counterclockwise direction, resulting in a run. However, when the flagella rotate in a clockwise direction, the flagella are no longer bundled, resulting in tumbles.
A diagram showing the run and tumble motion of bacteria. In the run, the bundeled flagella move in a counter clockwise rotation and the cell moves in a straight line. In the tumble, the flagella separate due to a clockwise rotation and the bacterial cell floats with no particular direction. This is followed by another run. If there is an attractant (a chemical gradient) the bacterial cell moves towards the attractant and the length of the run is extended.
Without a chemical gradient, flagellar rotation cycles between counterclockwise (run) and clockwise (tumble) with no overall directional movement. However, when a chemical gradient of an attractant exists, the length of runs is extended, while the length of tumbles is decreased. This leads to chemotaxis: an overall directional movement toward the higher concentration of the attractant.
  • What is the peptidoglycan layer and how does it differ between gram-positive and gram-negative bacteria?
  • Compare and contrast monotrichous, amphitrichous, lophotrichous, and peritrichous flagella.

Key concepts and summary

  • Prokaryotic cells differ from eukaryotic cells in that their genetic material is contained in a nucleoid rather than a membrane-bound nucleus. In addition, prokaryotic cells generally lack membrane-bound organelles.
  • Prokaryotic cells of the same species typically share a similar cell morphology and cellular arrangement .
  • Most prokaryotic cells have a cell wall that helps the organism maintain cellular morphology and protects it against changes in osmotic pressure.
  • Outside of the nucleoid, prokaryotic cells may contain extrachromosomal DNA in plasmids .
  • Prokaryotic ribosomes that are found in the cytoplasm have a size of 70S.
  • Some prokaryotic cells have inclusions that store nutrients or chemicals for other uses.
  • Some prokaryotic cells are able to form endospores through sporulation to survive in a dormant state when conditions are unfavorable. Endospores can germinate , transforming back into vegetative cells when conditions improve.
  • In prokaryotic cells, the cell envelope includes a plasma membrane and usually a cell wall.
  • Bacterial membranes are composed of phospholipids with integral or peripheral proteins. The fatty acid components of these phospholipids are ester-linked and are often used to identify specific types of bacteria. The proteins serve a variety of functions, including transport, cell-to-cell communication, and sensing environmental conditions. Archaeal membranes are distinct in that they are composed of fatty acids that are ether-linked to phospholipids.
  • Some molecules can move across the bacterial membrane by simple diffusion, but most large molecules must be actively transported through membrane structures using cellular energy.
  • Prokaryotic cell walls may be composed of peptidoglycan (bacteria) or pseudopeptidoglycan (archaea).
  • Gram-positive bacterial cells are characterized by a thick peptidoglycan layer, whereas gram-negative bacterial cells are characterized by a thin peptidoglycan layer surrounded by an outer membrane.
  • Some prokaryotic cells produce glycocalyx coatings, such as capsules and slime layers , that aid in attachment to surfaces and/or evasion of the host immune system.
  • Some prokaryotic cells have fimbriae or pili , filamentous appendages that aid in attachment to surfaces. Pili are also used in the transfer of genetic material between cells.
  • Some prokaryotic cells use one or more flagella to move through water. Peritrichous bacteria, which have numerous flagella, use runs and tumbles to move purposefully in the direction of a chemical attractant.

True/false

Bacteria have 80S ribosomes each composed of a 60S large subunit and a 40S small subunit.

False

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Fill in the blank

Prokaryotic cells that are rod-shaped are called _____________.

bacilli

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The type of inclusion containing polymerized inorganic phosphate is called _____________.

volutin (or metachromatic granule)

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Short answer

What is the direction of water flow for a bacterial cell living in a hypotonic environment? How do cell walls help bacteria living in such environments?

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How do bacterial flagella respond to a chemical gradient of an attractant to move toward a higher concentration of the chemical?

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Label the parts of the prokaryotic cell.

A diagram of a bacterial cell. The thick outer structure of the cell is not lableled. The next layer in (a thinner structure) is labeled E. A much thinner structure inside of that is labeled F. Inside of F is the main body of the cell. Small dots are labeled B. A long line forming a loop is labeled C. On the outside of the cell, short projectsions are labeled A and a long projection is labeled D.
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Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
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