Flagella and Cilia

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This transmission electron micrograph shows a cross section of nine microtubule doublets that form a hollow tube. Another microtubule doublet sits in the center of the tube.
This transmission electron micrograph of two flagella shows the microtubules’ 9 + 2 array: nine microtubule doublets surround a single microtubule doublet. (credit: modification of work by Dartmouth Electron Microscope Facility, Dartmouth College; scale-bar data from Matt Russell)

Flagella and Cilia (OpenStax Biology 2e)

The flagella (singular = flagellum) are long, hair-like structures that extend from the plasma membrane and enable an entire cell to move (for example, sperm, Euglena, and some prokaryotes). When present, the cell has just one flagellum or a few flagella. However, when cilia (singular = cilium) are present, many of them extend along the plasma membrane’s entire surface. They are short, hair-like structures that move entire cells (such as paramecia) or substances along the cell’s outer surface (for example, the cilia of cells lining the Fallopian tubes that move the ovum toward the uterus, or cilia lining the cells of the respiratory tract that trap particulate matter and move it toward your nostrils.)

Despite their differences in length and number, flagella and cilia share a common structural arrangement of microtubules called a “9 + 2 array.” This is an appropriate name because a single flagellum or cilium is made of a ring of nine microtubule doublets, surrounding a single microtubule doublet in the center.

– What is the main function of flagellum?

– What is a motile filamentous extracellular projection of eukaryotic cells and composed of both motile cilia and flagella?

Most bacteria propel themselves to food sources and other sites of interest using whip-like molecular propellers known as flagella. However, one research proposes members of the bacterial class Gammaproteobacteria eject their flagella when nutrients are scarce. These findings suggest a previously unknown mechanism for microbes to save energy in lean times. Earlier research has shown that bacteria switch from a mobile to stationary phase in the face of nutrient depletion, but it has been unclear how they deactivate their large, energy-intensive flagella.

Related Topic: The Flagella Staining


Clark, M., Douglas, M., Choi, J. Biology 2e. Houston, Texas: OpenStax. Access for free at: https://openstax.org/details/books/biology-2e




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Flagella-Driven Motility of Bacteria