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An undulipodium or undulopodium (Greek: "swinging foot"; plural undulipodia), or a 9+2 organelle is a motile filamentous extracellular projection of eukaryotic cells.It is basically synonymous to flagella and cilia which are differing terms for similar molecular structures used on different types of cells, and usually correspond to different waveforms.
Archaeal flagella are superficially similar to bacterial flagella in that it also has a rotary motor, but are different in many details and considered non-homologous. [18] [19] [20] Eukaryotic flagella—those of animal, plant, and protist cells—are complex cellular projections that lash back and forth.
Inside a cilium and a flagellum is a microtubule-based cytoskeleton called the axoneme. The axoneme of a primary cilium typically has a ring of nine outer microtubule doublets (called a 9+0 axoneme), and the axoneme of a motile cilium has two central microtubules in addition to the nine outer doublets (called a 9+2 axoneme).
Microtubule arrangement in a 9+2 axoneme of bronchiolar cilia. Microtubule-organizing centers function as the site where microtubule formation begins, as well as a location where free-ends of microtubules attract to. [2] Within the cells, microtubule-organizing centers can take on many different forms.
Schematic of the eukaryotic flagellum. 1-axoneme, 2-cell membrane, 3-IFT (Intraflagellar transport), 4-Basal body, 5-Cross section of flagellum, 6-Triplets of microtubules of basal body. Longitudinal section through the flagella area in Chlamydomonas reinhardtii. In the cell apex is the basal body that is the anchoring site for a flagellum.
The latter formation is commonly referred to as a "9+2" arrangement, wherein each doublet is connected to another by the protein dynein. As both flagella and cilia are structural components of the cell, and are maintained by microtubules, they can be considered part of the cytoskeleton. There are two types of cilia: motile and non-motile cilia.
Flagellins are a family of proteins present in flagellated bacteria [1] which arrange themselves in a hollow cylinder to form the filament in a bacterial flagellum. Flagellin has a mass on average of about 40,000 daltons. [2] [3] Flagellins are the principal component of bacterial flagella that have a crucial role in bacterial motility.
Bacterial flagella are helical filaments, each with a rotary motor at its base which can turn clockwise or counterclockwise. [16] [17] [18] They provide two of several kinds of bacterial motility. [19] [20] Archaeal flagella are called archaella, and function in much the same way as bacterial flagella