An undulipodium (a Greek word meaning "swinging foot") or a 9+2 organelle is a motile filamentous extracellular projection of eukaryotic cells. It is basically synonymous to flagella and cilia of the eukaryotic cells. In fact the name was coined to differentiate from the homologous structures present in prokaryotic cells. It is structurally a complex of microtubules along with motor proteins. The usage of the term is most vocally supported by Lynn Margulis, especially in support of her endosymbiotic theory. The eukaryotic cilia are structurally identical to eukaryotic flagella, although distinctions are sometimes made according to function and/or length.
Motile (or secondary) cilia are more numerous, with multiple cilia per cell, move in a wave-like action, and are responsible for movement in organisms such as ciliates and platyhelminthes, but also move extracellular substances in animals, such as the ciliary escalator found in the respiratory tract of mammals and the corona of rotifers.
Undulipodia are an extension of the cell membrane containing both cytoplasm and a regular arrangement of microtubules known as an axoneme. At the base of the extension lies a structure called the kinetosome or basal body which is attached via motor proteins to the microtubules. The kinetosome mediates movement through a chemical reaction, causing the microtubules to slide against one another and the whole structure to bend.
Biologists such as Margulis strongly advocate the use of the name, because of the apparent structural and functional differences between the cilia and flagella of prokaryotic and eukaryotic cells. They argue that the name flagella should be restricted only to prokaryotic organelles, such as bacterial flagella and spirochaete axial filaments. However, the term is not generally endorsed by most biologists because it is argued that the original purpose of the name does not sufficiently differentiate the cilia and flagella of eukaryotic from those of prokaryotic cells. For example, the early concept was the trivial homology of flagella of flagellates and pseudopodia of rhizopods. The consensus terminology is the use of cilium and flagellum for all purposes.
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