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| Flagella and Cilia |
Although the term “flagellum” is used in reference to both prokaryotes (archaea and bacteria) and eukaryotes (fungi, protists, plants, and animals), the structure and mechanism of action of this structure in prokaryotes are quite different from the structure and mechanism of action in eukaryotes.
Eukaryotic flagella and cilia, however, are structurally and functionally identical. The differences between them are in their number, length, and position. Flagella are less numerous, longer, and usually polar, while cilia are more numerous and shorter, covering much of the cell’s surface.
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Because the dividing line between eukaryotic flagella and cilia is not precise, many scientists use the term undulipodia as a collective word for both eukaryotic flagella and cilia. In some algae, other protists, and the gametes of certain plants with motile sperm, flagella and cilia can occur.
Bacterial Flagella
Bacterial flagella are composed of a single protein called flagellin. Molecules of this globular protein are stacked to form a thin filament approximately 0.01-0.015 micrometer in diameter. The filament protrudes through the cell wall at the tip of the bacterium (polar flagella) or over the entire surface of the bacterium (peritrichous flagella).
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| Bacterial flagella |
Spirochetes are unusual in this regard because the flagella do not pierce the cell wall but are located in the space between it and the plasma membrane. The base of each flagellum connects to a rotary motor anchored in the plasma membrane.
The cell provides energy to the motor, which then rotates the flagellum to allow cell movement. Rotational movement may be counter-clockwise, which leads to generally straight-line motion, or clockwise, which leads to a more random tumbling motion.
Eukaryotic Flagella and Cilia
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| Eukaryotic Flagella and Cilia |
Eukaryotic flagella and cilia, or undulipodia, are more complex and larger (approximately 0.25 micrometer in diameter) than their prokaryotic counterparts. The main component of these eukaryotic structures is the microtubule; a long, cylindrical structure composed of tubulin proteins. In eukaryotic flagella and cilia, two central microtubules are surrounded by a circular arrangement of nine microtubule pairs.
Eukaryotic flagella and cilia also contain more than five hundred other proteins, including dynein and kinesin, motor proteins that use cell energy to slide the microtubules past each other, causing an undulating motion (hence the name undulipodium). Unlike bacterial flagella, eukaryotic flagella and cilia are considered to be intracellular structures because they are covered by a continuation of the plasma membrane.
Although absent from fungi, undulipodia are found inmany protists and in some plants. Unicellular algae (such as Chlamydomonas and Euglena) and colonial algae (Volvox) use undulipodia for locomotion. Multicellular algae (Phaeophyta, Rhodophyta) produce flagellated sperm.
Among the true plants, bryophytes (Hepatophyta, Anthocerotophyta, and Bryophyta), ferns and their allies (Psilotophyta, Lycophyta, Sphenophyta, and Pterophyta), and some gymnosperms (Cycadophyta and Ginkgophyta) also produce flagellated sperm. Other gymnosperms (Coniferophyta) and angiosperms (Anthophyta)do not produce cells with flagella or cilia.
