3.3 Unique characteristics of prokaryotic cells  (Page 12/20)

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] ).

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.

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