An undulipodium or undulopodium (a
Greek word meaning "swinging foot"), or a 9+2 organelle is a motile filamentous extracellular projection of
eukaryotic cells. It is basically
synonymous
A synonym is a word, morpheme, or phrase that means exactly or nearly the same as another word, morpheme, or phrase in a given language. For example, in the English language, the words ''begin'', ''start'', ''commence'', and ''initiate'' are all ...
to
flagella
A flagellum (; ) is a hairlike appendage that protrudes from certain plant and animal sperm cells, and from a wide range of microorganisms to provide motility. Many protists with flagella are termed as flagellates.
A microorganism may have f ...
and
cilia
The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike projecti ...
which are differing terms for similar molecular structures used on different types of cells, and usually correspond to different
waveforms.
The name was coined to differentiate from the
analogous structures present in
prokaryotic cells. It is structurally a complex of
microtubules along with
motor proteins.
The usage of the term was early supported by
Lynn Margulis, especially in support of
endosymbiotic theory
Symbiogenesis (endosymbiotic theory, or serial endosymbiotic theory,) is the leading evolutionary theory of the origin of eukaryotic cells from prokaryotic organisms. The theory holds that mitochondria, plastids such as chloroplasts, and possibl ...
. The
eukaryotic cilia are structurally identical to eukaryotic flagella, although distinctions are sometimes made according to function and/or length.
[
] The
Gene Ontology database does not make a distinction between the two, referring to most undulipodium as "motile cilium", and to that in the
sperm
Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum, whi ...
as ''sperm flagellum''.
Structure
Undulipodia use a whip-like action to create movement of the whole cell, such as the movement of
sperm
Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum, whi ...
in the reproductive tract, and also create water movement as in the
choanocytes of
sponges.
''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
rotifer
The rotifers (, from the Latin , "wheel", and , "bearing"), commonly called wheel animals or wheel animalcules, make up a phylum (Rotifera ) of microscopic and near-microscopic pseudocoelomate animals.
They were first described by Rev. John H ...
s.
''Primary'' cilia function as sensory
antennae,
but are not ''undulipodia'' as primary cilia do not have the rotary movement mechanism found in
motile cilia.
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 protein
Motor proteins are a class of molecular motors that can move along the cytoplasm of cells. They convert chemical energy into mechanical work by the hydrolysis of ATP. Flagellar rotation, however, is powered by a proton pump.
Cellular functions ...
s 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.
Usage
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
A flagellate is a cell or organism with one or more whip-like appendages called flagella. The word ''flagellate'' also describes a particular construction (or level of organization) characteristic of many prokaryotes and eukaryotes and their ...
and
pseudopodia
A pseudopod or pseudopodium (plural: pseudopods or pseudopodia) is a temporary arm-like projection of a eukaryotic cell membrane that is emerged in the direction of movement. Filled with cytoplasm, pseudopodia primarily consist of actin filament ...
of
rhizopods
An amoeba (; less commonly spelled ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of Cell (biology), cell or unicellular organism with the ability to alter its shape, primarily by extending and ret ...
. The consensus terminology is the use of cilium and flagellum for all purposes.
References
{{Organelles
Cell anatomy
Cell movement