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] ).
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.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.
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?
